<p>To place this protractor or any measuring tool at a difficult to see point use a stylus such as a pencil or an awl or the metal tip of a compass. Place the point of the stylus on the given point and slide the protractor's two beams against it so that it rests at their intersection.</p>
<p>If the stylus is too thick you may need to angle it away from the protractor beam edges so that the side touching the beam aligns exactly with the point.</p>
<p>As Chris H explains, the measured angle is the same even if the center point is offset from the origin since the available edge and the baseline of the tool's center point are parallel.</p>

CC BY-SA 4.0
2020-12-09T20:46:24.977Z
I think you mean instead of center mark (which is under the knob) the inside corner of the protractor? That does sound like a good method to align it.
StayOnTarget
CC BY-SA 4.0
2020-12-09T20:51:10.480Z
I see, adjusted answer accordingly.
rebusB
CC BY-SA 4.0
2020-12-09T20:45:10.807Z
2020-12-09T20:48:39.390Z
2020-12-09T20:48:39.390Z
rebusB
<p>The angle between the ruler part and the the semicircular part is the same as the angle indicated on the scale.  This is because the centreline of the ruler is parallel to its edge.</p>
<p>In more detail:</p>
<p><a href="https://i.sstatic.net/XBVXA.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/XBVXA.png" alt="Parallels" /></a>
<sub>Image from <a href="https://commons.wikimedia.org/wiki/File:Parallel_transversal.svg" rel="nofollow noreferrer">Wikimedia Commons</a> user <a href="https://commons.wikimedia.org/wiki/User:Krishnavedala" rel="nofollow noreferrer">Krishnavedala</a></sub></p>
<p>Consider line <em>a</em> to be the top edge of the ruler in the picture and line <em>b</em> the centreline.  Line <em>t</em>, the transversal, is the base of the semicircle.  The top right <em>θ</em> where <em>a</em> and <em>t</em> meet is the angle you want to measure.  The bottom left <em>θ</em> is the angle the pointer makes with the semicircle.</p>
<p>This is a useful protractor for setting an angle or measuring a physical object with some thickness. For work purely on paper I'd prefer the clear plastic sort, marking in pencil and then using a steel rule for cutting if necessary.</p>
<p>Say you want to measure the interior angle of the parallelogram shown in freehand blue in this picture (imagine it drawn on paper under the ruler).
<a href="https://i.sstatic.net/2VvQg.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/2VvQg.jpg" alt="OP's photo marked up" /></a></p>
<p>You align the protractor as shown, so the edges highlighted in thin red  run along the lines you're interested in.  Then the other angle in red is what you read off.  This is a copy of the first red angle, rotated and enlarged (hence thicker), but not distorted.  You can see that this runs through the pivot and point to the pointer, aligned with its groove.  NB: Somehow I've actually placed the lower red angle in slightly the wrong place, aligned with the base of the (slightly more than) semicircular part instead of the zero line.</p>

CC BY-SA 4.0
2020-12-08T13:31:28.490Z
So, you think its more for measuring an exterior angle than an interior one? (even on a flat surface, like paper)
StayOnTarget
CC BY-SA 4.0
2020-12-08T13:34:51.057Z
On paper you can easily measure an interior angle, though you're effectively measuring the corresponding exterior angle and subtracting from 180°.  I'm marling up your photo now
Chris H
CC BY-SA 4.0
2020-12-08T13:29:26.643Z
2022-03-09T13:27:07.787Z
2022-03-09T13:27:07.787Z
Chris H
<p>I have this protractor:</p>
<p><a href="https://i.sstatic.net/aN8z2.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/aN8z2.jpg" alt="enter image description here" /></a></p>
<p>It seems to be the same or similar as this one: <a href="http://www.robinson.com.my/usr/product.aspx?pgid=4&amp;id=99&amp;grpid=29" rel="nofollow noreferrer">Robinson 5001# Stainless Steel Protractor
RBS-R-5001</a> but there are no instructions there.</p>
<p>The main problem with this protractor is that the 'origin' swivel point can't be accurately placed over the object you are trying to measure because you can't see where it is. For instance if you put it over a paper image of a triangle to measure an interior angle you can line up the tip of the protractor (small detent) but not the swivel point.</p>
<p>Also I don't see how you could use this e.g. to measure the interior angle of a real object as you would with a bevel gauge. So it does seem intended to be used for flat measurements, I just can't see how.</p>

CC BY-SA 4.0
2020-12-10T22:56:11.513Z
Could you explain that two or three different ways, please?

If the 'origin' swivel point can't be accurately placed, what else could be accurate?

If you're really Asking how the protractor could be accurately placed isn't that a very different Question?
Robbie Goodwin
<tool-usage>
How to use a protractor if you can't see what you are measuring?
StayOnTarget
# How to use a protractor if you can't see what you are measuring?

(Asked by: username_0 on 2020-12-08)

I have this protractor:

[![enter image description here](https://i.sstatic.net/aN8z2.jpg)](https://i.sstatic.net/aN8z2.jpg)

It seems to be the same or similar as this one: [Robinson 5001# Stainless Steel Protractor
RBS-R-5001](http://www.robinson.com.my/usr/product.aspx?pgid=4&id=99&grpid=29) but there are no instructions there.

The main problem with this protractor is that the 'origin' swivel point can't be accurately placed over the object you are trying to measure because you can't see where it is. For instance if you put it over a paper image of a triangle to measure an interior angle you can line up the tip of the protractor (small detent) but not the swivel point.

Also I don't see how you could use this e.g. to measure the interior angle of a real object as you would with a bevel gauge. So it does seem intended to be used for flat measurements, I just can't see how.

**Tags:** tool-usage

> **Comments:**
> **username_1**: Could you explain that two or three different ways, please?  If the 'origin' swivel point can't be accurately placed, what else could be accurate?  If you're really Asking how the protractor could be accurately placed isn't that a very different Question?  

---

## 2 answers

### Answer by username_3 on 2020-12-08. Score: 12 | Accepted answer

The angle between the ruler part and the the semicircular part is the same as the angle indicated on the scale. This is because the centreline of the ruler is parallel to its edge.

In more detail:

[![Parallels](https://i.sstatic.net/XBVXA.png)](https://i.sstatic.net/XBVXA.png)
Image from [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Parallel_transversal.svg) user [Krishnavedala](https://commons.wikimedia.org/wiki/User:Krishnavedala)

Consider line *a* to be the top edge of the ruler in the picture and line *b* the centreline. Line *t*, the transversal, is the base of the semicircle. The top right *θ* where *a* and *t* meet is the angle you want to measure. The bottom left *θ* is the angle the pointer makes with the semicircle.

This is a useful protractor for setting an angle or measuring a physical object with some thickness. For work purely on paper I'd prefer the clear plastic sort, marking in pencil and then using a steel rule for cutting if necessary.

Say you want to measure the interior angle of the parallelogram shown in freehand blue in this picture (imagine it drawn on paper under the ruler).
[![OP's photo marked up](https://i.sstatic.net/2VvQg.jpg)](https://i.sstatic.net/2VvQg.jpg)

You align the protractor as shown, so the edges highlighted in thin red run along the lines you're interested in. Then the other angle in red is what you read off. This is a copy of the first red angle, rotated and enlarged (hence thicker), but not distorted. You can see that this runs through the pivot and point to the pointer, aligned with its groove. NB: Somehow I've actually placed the lower red angle in slightly the wrong place, aligned with the base of the (slightly more than) semicircular part instead of the zero line.

> **Comments:**
> **username_0**: So, you think its more for measuring an exterior angle than an interior one? (even on a flat surface, like paper)  
> **username_3**: On paper you can easily measure an interior angle, though you're effectively measuring the corresponding exterior angle and subtracting from 180°.  I'm marling up your photo now  

---

### Answer by username_2 on 2020-12-09. Score: 2

To place this protractor or any measuring tool at a difficult to see point use a stylus such as a pencil or an awl or the metal tip of a compass. Place the point of the stylus on the given point and slide the protractor's two beams against it so that it rests at their intersection.

If the stylus is too thick you may need to angle it away from the protractor beam edges so that the side touching the beam aligns exactly with the point.

As username_3 explains, the measured angle is the same even if the center point is offset from the origin since the available edge and the baseline of the tool's center point are parallel.

> **Comments:**
> **username_0**: I think you mean instead of center mark (which is under the knob) the inside corner of the protractor? That does sound like a good method to align it.  
> **username_2**: I see, adjusted answer accordingly.  

---

