The Star user interface adheres rigorously to a small set of
principles designed to make the system seem friendly by
simplifying the human-machine interface.
Reprinted from Byte, issue 4/1982, pp. 242-282.
In April 1981, Xerox announced the 8010 Star Information System, a new
personal computer designed for offices. Consisting of a processor, a large
display, a keyboard, and a cursor-control device (see photo 1), it is intended
for business professionals who handle information.
|Photo 1: A Star work station showing the processor, display, keyboard and mouse.|
Star is a multifunction system combining document creation, data processing,
and electronic filing, mailing, and printing. Document creation includes text
editing and formatting, graphics editing, mathematical formula editing, and
page layout. Data processing deals with homogeneous, relational databases that
can be sorted, filtered, and formatted under user control. Filing is an
example of a network service utilizing the Ethernet local-area network (see
references 9 and 13). Files may be stored on a work station’s disk, on
a file server on the work station’s network, or on a file server on
a different network. Mailing permits users of work stations to communicate with
one another. Printing utilizes laser-driven raster printers capable of printing
both text and graphics.
As Jonathan Seybold has written, “This is a very different product:
Different because it truly bridges word processing and typesetting functions;
different because it has a broader range of capabilities than anything
which has preceded it; and different because it introduces to the commercial
market radically new concepts in human engineering.” (See reference 15.)
The Star user interface adheres rigorously to a small set of design principles.
These principles make the system seem familiar and friendly, simplify the
human-machine interaction, unify the nearly two dozen functional areas of Star,
and allow user experience in one area to apply in others. In reference 17,
we presented an overview of the features in Star. Here, we describe the principles
behind those features and illustrate the principles with examples. This
discussion is addressed to the designers of other computer programs and systems –
large and small.
Before describing Star’s user interface, several essential aspects of the
Star architecture should be pointed out. Without these elements, it would have
been impossible to design an interface anything like the present one.
The Star hardware was modeled after the experimental Xerox Alto computer (see reference
19). Like Alto, Star consists of a Xerox-developed, high-bandwidth, MSI
(medium-scale integration) processor; local disk storage; a bit-mapped display
screen having a 72-dots-per-inch resolution; a pointing device called the
“mouse”; and a connection to the Ethernet network. Stars are
higher-performance machines than Altos, being about three times as fast,
having 512K bytes of main memory (versus 256K bytes on most Altos), 10
or 29 megabytes of disk memory (versus 2.5 megabytes), a 10½- by 13frac12;-inch
display screen (versus 10frac12; by 8 inches), and a 10-megabits-per-second Ethernet
(versus 3 megabits). Typically, Stars, like Altos, are linked via Ethernets
to each other and to shared file, mail, and print servers. Communication
servers connect Ethernets to one another either directly or over telephone lines,
enabling internetwork communication. (For a detailed description of the Xerox
Alto computer, see the September 1981 BYTE article “The Xerox Alto Computer”
by Thomas A. Wadlow on page 58.)
The most important ingredient of the user interface is the bit-mapped display
screen. Both Star and Alto devote a portion of main memory to the screen: 100K bytes
in Star, 50K bytes (usually) in Alto. Every screen dot can be individually turned
on or off by setting or resetting the corresponding bit in memory. It should
be obvious that this gives both computers an excellent ability to portray visual
images. We believe that all impressive office systems of the future will have
bit-mapped displays. Memory cost will soon be insignificant enough that they will
be feasible even in home computers. Visual communication is effective, and it
can’t be exploited without graphics flexibility.
There must be a way to change dots on the screen quickly. Star has a high memory
bandwidth, about 90 megahertz (MHz). The entire Star screen is repainted from
memory 39 times per second, about a 50-MHz data rate between memory and the
screen. This would swamp most computer memories. However, since Star’s
memory is double-ported, refreshing the display does not appreciably slow down
processor memory access. Star also has separate logic devoted solely to
refreshing the display. Finally, special microcode has been written to assist
in changing the contents of memory quickly, permitting a variety of screen processing
that would not otherwise be practical (see reference 8).
People need a way to quickly point to items on the screen. Cursor step keys
are too slow; nor are they suitable for graphics. Both Star and Alto use a
pointing device called the mouse (see photo 2). First developed at Stanford
Research Institute (see reference 6), Xerox’s version has a ball on the
bottom that turns as the mouse slides over a flat surface such as a table.
Electronics sense the ball rotation and guide a cursor on the screen in
corresponding motions. The mouse possesses several important attributes:
|Photo 2: The Star keyboard and mouse. Note the two buttons on top of the mouse.|
|It is a “Fitts’s law” device. That is, after some practice you
can point with a mouse as quickly and easily as you can with the tip of your
finger. The limitations on pointing speed are those inherent in the human nervous
system (see references 3 and 7).
|It stays where it was left when you are not touching it. It doesn’t have
to be picked up like a light pen or stylus.
|It has buttons on top that can be sensed under program control. The buttons
let you point to and interact with objects on the screen in a variety of ways.
Every Star and Alto has its own hard disk for local storage of programs and
data. This enhances their personal nature, providing consistent access to
information regardless of how many other machines are on the
network or what anyone else is doing. Larger programs can be written, using
the disk for swapping.
The Ethernet lets both Stars and Altos have a distributed architecture.
Each machine is connected to an Ethernet. Other machines on the Ethernet are
dedicated as “servers” – machines that are attached to a
resource and provide access to that resource.
Star Design Methodology
We have learned from Star the importance of formulating the fundamental
concepts (the user’s conceptual model) before software is written,
rather than tacking on a user interface afterward. Xerox devoted
about thirty work-years to the design of the Star user interface. It
was designed before the functionality of the system was fully decided. It
was even designed before the computer hardware was built. We worked
for two years before we wrote a single line of actual product software. Jonathan
Seybold put it this way, “Most system design efforts start with hardware
specifications, follow this with a set of functional specifications for
the software, then try to figure out a logical user interface and command
structure. The Star project started the other way around: the paramount concern
was to define a conceptual model of how the user would relate to the system.
Hardware and software followed from this.” (See reference 15.)
In fact, before we even began designing the model, we developed a
methodology by which we would do the design. Our methodology report (see reference
One of the most troublesome and least understood aspects of interactive systems is
the user interface. In the design of user interfaces, we are
concerned with several issues: the provision of languages by which users can
express their commands to the computer; the design of display representations that
show the state of the system to the user; and other more abstract issues that
affect the user’s understanding of the system’s behavior. Many
of these issues are highly subjective and are therefore often addressed in an ad
hoc fashion. We believe, however, that more rigorous approaches to user
interface design can be developed...
These design methodologies are all unsatisfactory for the same basic reason: they
all omit an essential step that must precede the design of any successful
user interface, namely task analysis. By this we mean the analysis of the task
performed by the user, or users, prior to introducing the proposed computer
system. Task analysis involves establishing who the users are, what their
goals are in performing the task, what information they use in performing it,
what information they generate, and what methods they employ. The descriptions of
input and output information should include an analysis of the various objects,
or individual types of information entity, employed by the user...
The purpose of task analysis is to simplify the remaining stages in user interface
design. The current task description, with its breakdown of the information
objects and methods presently employed, offers a starting point for the definition
of a corresponding set of objects and methods to be provided by the computer system.
The idea behind this phase of design is to build up a new task environment for
the user, in which he can work to accomplish the same goals as before, surrounded
now by a different set of objects, and employing new methods.
Prototyping is another crucial element of the design process. System designers
should be prepared to implement the new or difficult concepts and then to throw
away that code when doing the actual implementation. As Frederick Brooks says,
the question “is not whether to build a pilot system and throw it away.
You will do that. The only question is whether to plan in advance to
build a throwaway, or to promise to deliver the throwaway to customers... Hence
plan to throw one away; you will, anyhow.” (See reference 2.) The Alto
served as a valuable prototype for Star. Over a thousand Altos were eventually
built. Alto users have had several thousand work-years of experience with them
over a period of eight years, making Alto perhaps the
largest prototyping effort ever. Dozens of experimental programs were written for
the Alto by members of the Xerox Palo Alto Research Center. Without the creative
ideas of the authors of those systems, Star in its present form would have been
impossible. In addition, we ourselves programmed various aspects of the Star
design on Alto, but all of it was “throwaway” code. Alto, with
its bitmapped display screen, was powerful enough to implement and test our
ideas on visual interaction.
Some types of concepts are inherently difficult for people to grasp. Without being
too formal about it, our experience before and during the Star design led
us to the following classification:
The characteristics on the left were incorporated into the Star user’s
conceptual model. The characteristics on the right we attempted to avoid.
The following main goals were pursued in designing the Star user interface:
|familiar user’s conceptual model
|seeing and pointing versus remembering and typing
|what you see is what you get
We will discuss each of these in turn.
Familiar User’s Conceptual Model
A user’s conceptual model is the set of concepts a person
gradually acquires to explain the behavior of a system, whether it be a computer
system, a physical system, or a hypothetical system. It is the model developed
in the mind of the user that enables that person to understand and interact
with the system. The first task for a system designer is to decide what model is
preferable for users of the system. This extremely important step is often
neglected or done poorly. The Star designers devoted several work-years at
the outset of the project discussing and evolving what we considered an
appropriate model for an office information system: the metaphor of a physical office.
The designer of a computer system can choose to pursue familiar
analogies and metaphors or to introduce entirely new functions requiring new
approaches. Each option has advantages and disadvantages. We decided to create
electronic counterparts to the physical objects in an office: paper, folders,
file cabinets, mail boxes, and so on – an electronic metaphor for the
office. We hoped this would make the electronic “world” seem more
familiar, less alien, and require less training. (Our initial experiences with
users have confirmed this.) We further decided to make the electronic analogues
be concrete objects. Documents would be more than file names on a disk;
they would also be represented by pictures on the display screen. They would
be selected by pointing to them with the mouse and clicking one of the
buttons. Once selected, they would be moved, copied, or deleted by
pushing the appropriate key. Moving a document became the electronic equivalent of
picking up a piece of paper and walking somewhere with it. To file a
document, you would move it to a picture of a file drawer, just as you
take a physical piece of paper to a physical file cabinet.
The reason that the user’s conceptual model should be decided first
when designing a system is that the approach adopted changes the functionality
of the system. An example is electronic mail. Most electronic-mail systems
draw a distinction between messages and files to be sent to other
people. Typically, one program sends messages and a different program handles file
transfers, each with its own interface. But we observed that offices make
no such distinction. Everything arrives through the mail, from one-page memos
to books and reports, from intraoffice mail to international mail. Therefore,
this became part of Star’s physical-office metaphor. Star users mail documents
of any size, from one page to many pages. Messages are short documents, just
as in the real world. User actions are the same whether the recipients are
in the next office or in another country.
A physical metaphor can simplify and clarify a system. In addition to
eliminating the artificial distinctions of traditional computers, it can eliminate
commands by taking advantage of more general concepts. For example, since
moving a document on the screen is the equivalent of picking up a piece
of paper and walking somewhere with it, there is no “send mail” command.
You simply move it to a picture of an out-basket. Nor is there a “receive
mail” command. New mail appears in the in-basket as it is received.
When new mail is waiting, an envelope appears in the picture of the
in-basket (see figure 1). This is a simple, familiar, nontechnical approach to
computer mail. And it’s easy once the physical-office metaphor is adopted!
|Figure 1: In-basket and out-basket icons. The in-basket contains an envelope indicating that mail has been received. (This figure was taken directly from the Star screen. Therefore, the text appears at screen resolution.)|
While we want an analogy with the physical world for familiarity, we don’t
want to limit ourselves to its capabilities. One of the raisons d’être
for Star is that physical objects do not provide people with enough power to
manage the increasing complexity of the “information age.” For example,
we can take advantage of the computer’s ability to search rapidly by providing
a search function for its electronic file drawers, thus helping to solve the
long-standing problem of lost files.
Every user’s initial view of Star is the “Desktop,” which
resembles the top of an office desk, together with surrounding furniture and
equipment. It represents your working environment – where your current
projects and accessible resources reside. On the screen are displayed pictures of
familiar office objects, such as documents, folders, file drawers, in-baskets,
and out-baskets. These objects are displayed as small pictures or “icons,”
as shown in figure 2.
|Figure 2: A Desktop as it appears on the Star screen. Several commonly used icons appear across the top of the screen, including documents to serve as “form-pad” sources for letters, memos, and blank paper. An open window displaying a document containing an illustration is also shown.|
You can “open” an icon to deal with what it represents. This
enables you to read documents, inspect the contents of folders and file
drawers, see what mail you have received, etc. When opened, an icon expands into a
larger form called a “window,” which displays the icon’s
contents. Windows are the principal mechanism for displaying and manipulating information.
The Desktop “surface” is displayed as a distinctive gray pattern.
This restful design makes the icons and windows on it stand out crisply,
minimizing eyestrain. The surface is organized as an array of one-inch squares,
14 wide by 11 high. An icon can be placed in any square, giving a maximum
of 154 icons. Star centers an icon in its square, making it easy to line
up icons neatly. The Desktop always occupies the entire display screen; even when
windows appear on the screen, the Desktop continues to exist “beneath” them.
The Desktop is the principal Star technique for realizing the physical-office
metaphor. The icons on it are visible, concrete embodiments of the corresponding
physical objects. Star users are encouraged to think of the objects on
the Desktop in physical terms. Therefore, you can move the icons around to arrange
your Desktop as you wish. (Messy Desktops are certainly possible, just as
in real life.) Two icons cannot occupy the same space (a basic law of physics).
Although moving a document to a Desktop resource such as a printer involves
transferring the document icon to the same square as the printer icon, the printer
immediately “absorbs” the document, queuing it for printing. You can leave
documents on your Desktop indefinitely, just as on a real desk, or you can
file them away in folders or file drawers. Our intention and hope is
that users will intuit things to do with icons, and that those things
will indeed be part of the system. This will happen if:
- Star models the real world accurately enough. Its similarity with
the office environment preserves your familiar way of working and your existing
concepts and knowledge.
- Sufficient uniformity is in the system. Star’s principles and
“generic” commands (discussed below) are applied throughout the
system, allowing lessons learned in one area to apply to others.
The model of a physical office provides a simple base from which learning can
proceed in an incremental fashion. You are not exposed to entirely new
concepts all at once. Much of your existing knowledge is embedded in the base.
In a functionally rich system, it is probably not possible to represent
everything in terms of a single model. There may need to be more than one
model. For example, Star’s records-processing facility cannot use the
physical-office model because physical offices have no “records
processing” worthy of the name. Therefore, we invented a different
model, a record file as a collection of fields. A record can be displayed as
a row in a table or as filled-in fields in a form. Querying is accomplished
by filling in a blank example of a record with predicates describing the
desired values, which is philosophically similar to Zloof’s
“Query-by-Example” (see reference 21).
Of course, the number of different user models in a system must be kept to
a minimum. And they should not overlap; a new model should be introduced
only when an existing one does not cover the situation.
Seeing and Pointing
A well-designed system makes everything relevant to a task visible on the
screen. It doesn’t hide things under CODE+key combinations or
force you to remember conventions. That burdens your memory. During conscious
thought, the brain utilizes several levels of memory, the most important being
the “short-term memory.” Many studies have analyzed the short-term
memory and its role in thinking. Two conclusions stand out: (1) conscious
thought deals with concepts in the short-term memory (see reference 1)
and (2) the capacity of the short-term memory is limited (see reference 14).
When everything being dealt with in a computer system is visible, the
display screen relieves the load on the
short-term memory by acting as a sort of “visual cache.” Thinking
becomes easier and more productive. A well-designed computer system can
actually improve the quality of your thinking (see reference 16).
In addition, visual communication is often more efficient than linear
communication; a picture is worth a thousand words.
A subtle thing happens
when everything is visible: the display becomes reality. The user model becomes
identical with what is on the screen. Objects can be understood purely in
terms of their visible characteristics. Actions can be
understood in terms of their effects on I the screen. This lets users conduct
experiments to test, verify, and expand their understanding –
the essence of experimental science.
In Star, we have tried to make the objects and actions in the system visible.
Everything to be dealt with and all commands and effects have a visible
representation on the display screen or on the keyboard. You never have to
remember that, for example, CODE+Q does something in one context and something different
in another context. In fact, our desire to eliminate this possibility led us to
abolish the CODE key. (We have yet to see a computer system with a CODE key
that doesn’t violate the principle of visibility.) You never invoke a
command or push a key and have nothing visible happen. At the very least,
a message is posted explaining that the command doesn’t work in this
context, or it is not implemented, or there is an error.. It is disastrous to
the user’s model when you invoke an action and the system does nothing
in response. We have seen people push a key several times in one system or
another trying to get a response. They are not sure whether the system has
“heard” them or not. Sometimes the system is simply throwing away
their keystrokes. Sometimes it is just slow and is queuing the keystrokes;
you can imagine the unpredictable behavior that is possible.
We have already mentioned icons and windows as mechanisms for making the
concepts in Star visible. Other such mechanisms are Star’s property
and option sheets. Most objects in Star have properties. A property sheet
is a two-dimensional, form-like environment that displays those properties.
Figure 3 shows the character property sheet. It appears on the screen
whenever you make a text selection and push the PROPERTIES key. It contains
such properties as type font and size; bold, italic, underline, and strikeout
face; and superscript/subscript positioning. Instead of having to remember
the properties of characters, the current settings of those properties, and,
worst of all, how to change those properties, property sheets simply show
everything on the screen. All the options are presented. To change one,
you point to it with the mouse and push a button. Properties in effect
are displayed in reverse video.
|Figure 3: The property sheet for text characters.|
This mechanism is used for all properties of all objects in
the system. Star contains a couple of hundred properties. To keep you from
being overwhelmed with information, property sheets display only the properties
relevant to the type of object currently selected (e.g., character, paragraph,
page, graphic line, formula element, frame, document, or folder). This is
an example of “progressive disclosure”: hiding complexity until it
is needed. It is also one of the clearest examples of how an emphasis on
visibility can reduce the amount of remembering and typing required.
Property sheets may be thought of as an alternate representation for objects.
The screen shows you the visible characteristics of objects, such as the type
font of text characters or the names of icons. Property sheets show you
the underlying structure of objects as they make this structure visible and accessible.
Invisibility also plagues the commands in some systems. Commands often have
several arguments and options that you must remember with no assistance from the
system. Star addresses this problem with option sheets (see figure 4),
a two-dimensional, form-like environment that displays the arguments to
commands. It serves the same function for command arguments that property sheets
do for object properties.
|Figure 4: The option sheet for the Find command showing both the Search and Substitute options. The last two lines of options appear only when CHANGE IT is turned on.|
What You See Is What You Get
“What you see is what you get” (or WYSIWYG) refers to the
situation in which the display screen portrays an accurate rendition of
the printed page. In systems having such capabilities as multiple fonts and
variable line spacing, WYSIWYG requires a bit-mapped display because only that has
sufficient graphic power to render those characteristics accurately.
WYSIWYG is a simplifying technique for document-creation systems. All composition is
done on the screen. It eliminates the iterations that plague users
of document compilers. You can examine the appearance of a page on
the screen and make changes until it looks right. The printed page
will look the same (see figure 5). Anyone who has used a document compiler
or post-processor knows how valuable WYSIWYG is. The first powerful WYSIWYG
editor was Bravo, an experimental editor developed for Alto at the Xerox Palo
Alto Research Center (see reference 12). The text-editor aspects of Star were
derived from Bravo.
|Figure 5: A Star document showing multicolumn text, graphics, and formulas. This is the way the document appears on the screen. It is also the way it will print (at higher resolution, of course).|
Trade-offs are involved in WYSIWYG editors, chiefly having to do with the
lower resolution of display screens. It is never possible to get an
exact representation of a printed page on the screen since most screens have only
50 to 100 dots per inch (72 in Star), while most printers have higher resolution.
Completely accurate character positioning is not possible. Nor is it
usually possible to represent shape differences for fonts smaller than eight points
in size since there are too few dots per character to
be recognizable. Even 10-point (“normal” size) fonts may be uncomfortably
small on the screen, necessitating a magnified mode for viewing text. WYSIWYG
requires very careful design of the screen fonts in order to keep text on
the screen readable and attractive. Nevertheless, the increase in productivity made
possible by WYSIWYG editors more than outweighs these difficulties.
Star has a few commands that can be used throughout the system: MOVE, COPY,
DELETE, SHOW PROPERTIES, COPY PROPERTIES, AGAIN, UNDO, and HELP. Each
performs the same way regardless of the type of object selected. Thus,
we call them “universal” or “generic” commands.
For example, you follow the same set of actions to move text in a document and
to move a line in an illustration or a document in a folder: select the
object, push the MOVE key, and indicate a destination.
(HELP and UNDO don’t use a selection.) Each generic command has
a key devoted to it on the keyboard.
These commands are far more basic than the commands in other computer systems. They
strip away the extraneous application-specific semantics to get at the underlying
principles. Star’s generic commands derive from fundamental computer-science
concepts because they also underlie operations in programming languages. For
example, much program manipulation of data structures involves moving or
copying values from one data structure to another. Since Star’s generic
commands embody fundamental underlying concepts, they are widely applicable.
Each command fills a variety of needs, meaning fewer commands are required.
This simplicity is desirable in itself, but it has another subtle advantage:
it makes it easy for users to form a model of the system. People can use
what they understand. Just as progress in science derives from simple, clear
theories, progress in the usability of computers is coming to depend on simple,
clear user interfaces.
MOVE is the most powerful command in the system. It is used during text editing
to rearrange letters in a word, words in a sentence, sentences in a paragraph,
and paragraphs in a document. It is used during graphics editing to move
picture elements, such as lines and rectangles, around in an illustration.
It is used during formula editing to move mathematical structures, such as
summations and integrals, around in an equation. It replaces the conventional
“store file” and “retrieve file” commands; you
simply move an icon into or out of a file drawer or folder. It eliminates
the “send mail” and “receive mail” commands; you
move an icon to an out-basket or from an in-basket. It replaces the “print”
command; you move an icon to a printer. And so on. MOVE strips away much
of the historical clutter of computer commands. It is more fundamental than the
myriad of commands it replaces. It is simultaneously more powerful and simpler.
Much simplification comes from Star’s object-oriented interface. The
action of setting properties also replaces a myriad of commands. For example,
changing paragraph margins is a command in many systems. In Star, you
do it by selecting a paragraph object and setting its MARGINS property. (For
more information on object-oriented languages, see the August 1981 BYTE.)
Consistency asserts that mechanisms should be used in the same way wherever
they occur. For example, if the left mouse button is used to select a
character, the same button should be used to select a graphic line or an
icon. Everyone agrees that consistency is an admirable goal. However,
it is perhaps the single hardest characteristic of all to achieve in a computer
system. In fact, in systems of even moderate complexity, consistency may not
be well defined.
A question that has defied consensus in Star is what should happen to
a document after it has been printed. Recall that a user prints a document
by selecting its icon, invoking MOVE, and designating a printer icon.
The printer absorbs the document, queuing it for printing. What happens to that
document icon after printing is completed? The two plausible alternatives are:
- The system deletes the icon.
- The system does not delete the icon, which leads to several further alternatives:
- The system puts the icon back where it came from (i.e., where it was
before MOVE was invoked).
- The system puts the icon at an arbitrary spot on the Desktop.
- The system leaves the icon in the printer. You must move it out
of the printer explicitly.
The consistency argument for the first alternative goes as follows: when you move
an icon to an out-basket, the system mails it and then deletes it from your
Desktop. When you move an icon to a file drawer, the system files it and
then deletes it from your Desktop. Therefore, when you move an icon to
a printer, the system should print it and then delete it from your Desktop.
Function icons should behave consistently with one another.
The consistency argument for the second alternative is: the user’s conceptual
model at the Desktop level is the physical-office metaphor. Icons
are supposed to behave similarly to their physical counterparts. It makes sense
that icons are deleted after they are mailed because after you put a
piece of paper in a physical out-basket and the mailperson picks it up, it
is gone. However, the physical analogue for printers is the office copier,
and there is no notion of deleting a piece of paper when you make a
copy of it. Function icons should behave consistently with their physical counterparts.
There is no one right answer here. Both arguments emphasize a dimension of
consistency. In this case, the dimensions happen to overlap. We eventually chose
alternative 2a for the following reasons:
- Model dominance – The physical metaphor is the stronger model at
the Desktop level. Analogy with physical counterparts does form the basis for
people’s understanding of what icons are and how they behave. Argument 1
advocates an implicit model that must be learned; argument 2 advocates
an explicit model that people already have when they are introduced
to the system. Since people do use their existing knowledge when confronted
with new situations, the design of the system should be based on that knowledge.
This is especially important if people are to be able to intuit new
uses for the features they have learned.
- Pragmatics – It is dangerous to delete things when users don’t
expect it. The first time a person labors over a document, gets it just right,
prints it, and finds that it has disappeared, that person is going to become
very nervous, not to mention angry. We also decided to put it back where
it came from (2a instead of 2b or 2c) for the pragmatic reason that this involves
slightly less work on the user’s part.
- Seriousness – When you file or mail an icon, it is not deleted
entirely from the system. It still exists in the file drawer or in the
recipients’ in-baskets. If you want it back, you can move it back out
of the file drawer or send a message to one of the recipients asking to have
a copy sent back. Deleting after printing, however, is final; if you move a
document to a printer and the printer deletes it, that document is gone for good.
One way to get consistency into a system is to adhere to paradigms for
operations. By applying a successful way of working in one area to other
areas, a system acquires a unity that is both apparent and real. Paradigms that Star
|Editing – Much of what you do in Star can be thought of as
editing. In addition to the conventional text, graphics, and formula
editing, you manage your files by editing filing windows. You arrange your
working environment by editing your Desktop. You alter properties by
editing property sheets. Even programming can be thought of as editing
data structures (see reference 16).
|Information retrieval – A lot of power can be gained by
applying information-retrieval techniques to information wherever it exists in
a system. Star broadens the definition of “database.” In addition
to the traditional notion as represented by its record files, Star views file
drawers as databases of documents, in-baskets as databases of mail, etc.
This teaches users to think of information retrieval as a general tool
applicable throughout the system.
|Copying – Star elevates the concept of “copying” to
a high level: that of a paradigm for creating. In all the various domains of
Star, you create by copying. Creating something out of nothing is a
difficult task. Everyone has observed that it is easier to modify an
existing document or program than to write it originally. Picasso once
said, “The most awful thing for a painter is the white canvas... To
copy others is necessary.” (See reference 20.) Star makes a serious
attempt to alleviate the problem of the “white canvas” by making
copying a practical aid to creation. For example, you create new icons
by copying existing ones. Graphics are created by copying existing graphic images
and modifying them. In a sense, you can even type characters in Star’s
216-character set by “copying” them
from keyboard windows (see figure 6).
These paradigms change the very way you think. They lead to new
habits and models of behavior that are more powerful and productive. They
can lead to a human-machine synergism.
|Figure 6: The keyboard-interpretation window serves as the source of characters that may be entered from the keyboard. The character set shown here contains a variety of office symbols.|
Star obtains additional consistency by using the class and subclass notions
of Simula (see reference 4) and Smalltalk (see reference 11). The clearest example
of this is classifying icons at a higher level into data icons and
function icons. Data icons represent objects on which actions are performed.
Currently, the three types (i.e., subclasses) of data icons are documents,
folders, and record files. Function icons represent objects that perform actions.
Function icons are of many types, with more being added as the system
evolves: file drawers, in- and out-baskets, printers, floppy-disk drives,
calculators, terminal emulators, etc.
In general, anything that can be done to one data icon can be done to all,
regardless of its type, size, or location. All data icons can be moved,
copied, deleted, filed, mailed, printed, opened, closed, and a variety of
other operations applied. Most function icons will accept any data icon; for
example, you can move any data icon to an out-basket. This use of the class
concept in the user-interface design reduces the artificial
distinctions that occur in some systems.
Simplicity is another principle with which no one can disagree. Obviously,
a simple system is better than a complicated one if they have the same
capabilities. Unfortunately, the world is never as simple as that.
Typically, a trade-off exists between easy novice use and efficient expert use.
The two goals are not always compatible. In Star, we have tried to follow
Alan Kay’s maxim: “simple things should be simple; complex things
should be possible.” To do this, it was sometimes necessary to make
common things simple at the expense of uncommon things being harder.
Simplicity, like consistency, is not a clear-cut principle.
One way to make a system appear simple is to make it uniform and consistent,
as we discussed earlier. Adhering to those principles leads to a simple
user’s model. Simple models are easier to understand and work
with than intricate ones.
Another way to achieve simplicity is to minimize the redundancy in a system.
Having two or more ways to do something increases the complexity without
increasing the capabilities. The ideal system would have a minimum of
powerful commands that obtained all the desired functionality and that did
not overlap. That was the motivation for Star’s “generic”
commands. But again the world is not so simple. General mechanisms are
often inconvenient for high-frequency actions. For example, the SHOW
PROPERTIES command is Star’s general mechanism for changing properties,
but if is too much of an interruption during typing. Therefore, we added
keys to optimize the changing of certain character properties: BOLD,
ITALICS, UNDERLINE, SUPERSCRIPT, SUBSCRIPT, LARGER/SMALLER (font), CENTER
(paragraph). These significantly speed up typing, but they don’t add
any new functionality. In this case, we felt the trade-off was worth
it because typing is a frequent activity. “Minimum redundancy” is
a good but not absolute guideline.
In general, it is better to introduce new general mechanisms by which
“experts” can obtain accelerators rather than add a lot of
special one-purpose-only features. Star’s mechanisms are discussed below
under “User Tailorability.”
Another way to have the system as a whole appear simple is to make each
of its parts simple. In particular, the system should avoid overloading
the semantics of the parts. Each part should be kept conceptually clean.
Sometimes, this may involve a major redesign of the user interface. An
example from Star is the mouse, which has been used on the Alto for eight
years. Before that, it was used on the NLS system at Stanford Research
Institute (see reference 5). All of those mice have three buttons on top.
Star has only two. Why did we depart from “tradition”? We observed
that the dozens of Alto programs all had different semantics for the mouse buttons.
Some used them one way, some another. There was no consistency between
systems. Sometimes, there was not even consistency within a system. For
example, Bravo uses the mouse buttons for selecting text, scrolling windows,
and creating and deleting windows, depending on where the cursor is when
you push a mouse button. Each of the three buttons has its own meaning in
each of the different regions. It is difficult to remember which button does what where.
Thus, we decided to simplify the mouse for Star. Since it is apparently quite
a temptation to overload the semantics of the buttons, we eliminated temptation
by eliminating buttons. Well then, why didn’t we use a one-button mouse?
Here the plot thickens. We did consider and prototype a one-button mouse
interface. One button is sufficient (with a little cleverness) to provide all
the functionality needed in a mouse. But when we tested the interface on
naive users, as we did with a variety of features, we found that they had a
lot of trouble making selections with it.
In fact, we prototyped and tested six different semantics for the mouse
buttons: one one-button, four two-button, and a three-button design. We
were chagrined to find that while some were better than others, none of
them was completely easy to use, even though, a priori, it seemed
like all of them would work! We then took the most successful features of
two of the two-button designs and prototyped and tested them as a seventh
design. To our relief, it not only tested better than any of the other six,
everyone found it simple and trouble-free to use.
This story has a couple of morals:
|The intuition of designers is error-prone, no matter how good or bad they are.
|The critical parts of a system should be tested on representative users, preferably
of the “lowest common denominator” type.
|What is simplest along any one dimension (e.g., number of buttons) is not necessarily
conceptually simplest for users; in particular, minimizing the number of
keystrokes may not make a system easier to use.
Larry Tesler defines a mode as follows:
A mode of an interactive computer system is a state of the user interface
that lasts for a period of time, is not associated with any particular
object, and has no role other than to place an interpretation on operator
input. (See reference 18.)
Many computer systems use modes because there are too few keys on the
keyboard to represent all the available commands. Therefore, the interpretation
of the keys depends on the mode or state the system is in. Modes can
and do cause trouble by making habitual actions cause unexpected results.
If you do not notice what mode the system is in, you may find yourself
invoking a sequence of commands quite different from what you had intended.
Our favorite story about modes, probably apocryphal, involves
Bravo. In Bravo, the main typing keys are normally interpreted as
commands. The “i” key invokes the Insert command, which puts
the system in “insert mode.” In insert mode, Bravo
interprets keystrokes as letters. The story goes that a person intended
to type the word “edit” into his document, but he forgot
to enter insert mode first. Bravo interpreted “edit” as the following commands:
|E(verything)||select everything in the document|
|I(nsert)||enter insert mode|
|t||type a “t”|
The entire contents of the document were replaced by the letter “t.”
This makes the point, perhaps too strongly, that modes should be
introduced into a user interface with caution, if at all.
Commands in Star take the form of noun-verb. You specify the object of
interest (the noun) and then invoke a command to manipulate it (the verb). Specifying
an object is called “making a selection.” Star provides
powerful selection mechanisms that reduce the number and complexity of commands
in the system. Typically, you will exercise more dexterity and judgment
in making a selection than in invoking a command. The object (noun) is
almost always specified before the action (verb) to be performed. This
helps make the command interface modeless; you can change your mind as
to which object to affect simply by making a new selection before invoking
the command. No “accept” function is needed to terminate
or confirm commands since invoking the command is the last step.
Inserting text does not even require a command; you simply make a selection and
begin typing. The text is placed after the end of the selection.
The noun-verb command form does not by itself imply that a command
interface is modeless. Bravo also uses the noun-verb form; yet, it is
a highly modal editor (although the latest version of Bravo has drastically
reduced its modalness). The difference is that Bravo tries to make
one mechanism (the main typing keys) serve more than one function (entering
letters and invoking commands). This inevitably leads to confusion. Star avoids
the problem by having special keys on the keyboard devoted solely to invoking
functions. The main typing keys only enter characters. (This is another
example of the simplicity principle: avoid overloading mechanisms with meanings.)
Modes are not necessarily bad. Some modes can be helpful by simplifying the
specification of extended commands. For example, Star uses a “field fill-in
order specification mode.” In this mode, you can specify the order in
which the NEXT key will step through the fields in the document. Invoking
the SET FILL-IN ORDER command puts the system in the mode. Each field
you now select is added to the fill-in order. You terminate the mode by
pushing the STOP key. Star also utilizes temporary modes as part of the
MOVE, COPY, and COPY PROPERTIES commands. For example, to move an object,
you select it, push the MOVE key that puts the system in “move mode,”
and then select the destination. These modes work for two reasons. First,
they are visible. Star posts a message in the Message Area at the top of
the screen indicating that a mode is in effect. The message remains there for
the duration of the mode. Star also changes the shape of
the cursor as an additional indication. You can always tell the state of the
system by inspection (see figure 7). Second, the allowable actions are
constrained during modes. The only action that is allowed – except
for actions directly related to the mode – is scrolling to another
part of the document. This constraint makes it even more apparent that the
system is in an unusual state.
|Figure 7: Some of the cursor shapes used by the Star to indicate the state of the system. The cursor is a 16- by 16-bit map that can be changed under program control.|
No matter how general or powerful a system is, it will never satisfy all
its potential users. People always want ways to speed up often-performed
operations. Yet, everyone is different. The only solution is to design
the system with provisions for user extensibility built in. The following
mechanisms are provided by Star:
|You can tailor the appearance of your system in a variety of ways. The
simplest is to choose the icons you want on your Desktop, thus tailoring your
working environment. At a more sophisticated level, a work station can be
purchased with or without certain functions. For example, not everyone may
want the equation facility. Xerox calls this “product factoring.”
|You can set up blank documents with text, paragraph, and page layout
defaults. For example, you might set up one document with the normal text
font being 10-point Classic and another with it being 12-point Modern italic.
The documents need not be blank; they may contain fixed text and graphics, and
fields for variable fill-in. A typical form might be a business-letter form
with address, addressee, salutation, and body fields, each field with its
own default text style. Or it might be an accounting form with lines
and tables. Or it might be a mail form with To, From, and Subject fields, and
a heading tailored to each individual. Whatever the form or document, you can put
it on your Desktop and make new instances of it by selecting it and invoking
COPY. Thus, each form can act like a “pad of paper” from which
new sheets can be “torn off.”
Interesting documents to set up are “transfer sheets,” documents containing
a variety of graphics symbols tailored to different applications. For example,
you might have a transfer sheet containing buildings in different sizes and shapes,
or one devoted to furniture, animals, geometric shapes, flowchart symbols,
circuit components, logos, or a hundred other possibilities. Each sheet would make
it easier to create a certain type of illustration. Graphics experts could even
construct the symbols on the sheets, so that users could create high-quality
illustrations without needing as much skill.
|You can tailor your filing system by changing the sort order in file
drawers and folders. You can also control the filing hierarchy by putting folders
inside folders inside folders, to any desired level.
|You can tailor your record files by defining any number of “views”
or them. Each view consists of a filter, a sort order, and a formatting
document. A filter is a set of predicates that produces a subset of the
record file. A formatting document is any document that contains fields whose
names correspond to those in the record file. Records are always displayed
through some formatting document; they have no inherent external representation.
Thus, you can set up your own individual subset(s) and appearance(s) for
a record file, even if the record file is shared by several users.
|You can define “meta operations” by writing programs in the CUStomer
Programming language CUSP. For example, you can further tailor your forms
by assigning computation rules expressed in CUSP to fields. Eventually, you
will be able to define your own commands by placing CUSP “buttons” into documents.
|You can define abbreviations for commonly used terms by means of the abbreviation
definition/expansion facility. For example, you might define “sdd”
as an abbreviation for “Xerox Systems Development Department.” The
expansion can be an entire paragraph, or even multiple paragraphs. This is handy if
you create documents out of predefined “boilerplate” paragraphs,
as the legal profession does. The expansion can even be an illustration or
|Every user has a unique name used for identification to the system,
usually the user’s full name. However, you can define one or more aliases
by which you are willing to be known, such as your last name only,
a shortened form of your name, or a nickname. This lets you personalize
your identification to the rest of the network.
In the 1980s, the most important factors affecting how prevalent computer usage
becomes will be reduced cost, increased functionality, improved availability and
servicing, and, perhaps most important of all, progress in user-interface
design. The first three alone are necessary, but not sufficient for widespread
use. Reduced cost will allow people to buy computers, but improved user interfaces
will allow people to use computers. In this article, we have presented some
principles and techniques that we hope will lead to better user interfaces.
User-interface design is still an art, not a science. Many times during the
Star design we were amazed at the depth and subtlety of user-interface issues,
even such supposedly straightforward issues as consistency and simplicity.
Often there is no one “right” answer. Much of the time there
is no scientific evidence to support one alternative over another, just
intuition. Almost always there are trade-offs. Perhaps by the end of the
decade, user-interface design will be a more rigorous process. We hope
that we have contributed to that progress.
About the Authors
These five Xerox employees have worked on the Star user interface project
for the past five years. Their academic backgrounds are in computer
science and psychology.
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