A compound modifier (also called a compound adjective, phrasal adjective, or adjectival phrase) is a compound of two or more attributive words: That is, more than one word that together modify a noun. Compound modifiers are grammatically equivalent to single-word modifiers, and can be used in conjunction with other modifiers.
The punctuation of compound modifiers in English depends on their grammatical role. Attributive compounds – modifiers within the noun phrase – are typically hyphenated, whereas the same compound used as a predicate will typically not be.
Compound adjectives 
Compound adjectives, like normal adjectives, modify noun phrases. Grammatically, there is no difference between hot metal and white-hot metal – the latter is a compound adjective because it is made of two words used in conjunction.
Note that not all sequences of adjectives (or other types of words) modifying a noun phrase are necessarily parts of one or more compound adjectives. White-hot metal and white hot metal refer to subtly different things: in the first, white modifies hot which modifies metal – it is this layering of modification that calls for the hyphenation in order to clarify the meaning, that the metal mentioned is very hot. In the second example, however, white and hot separately modify the noun – if one were to be removed, the other's relationship with the noun would be unchanged.
Hyphenation of elements 
Conventionally, and with the support of modern writing guides, compound modifiers that appear before a noun phrase may include a hyphen between each word, subject to a number of exceptions. Hyphens are used in this way to help prevent confusion; without their use, a reader might interpret the words separately, rather than as a phrase. One or more hyphens join the relevant words into a single idea.
Major style guides advise consulting a dictionary to determine whether a compound adjective should be hyphenated; the dictionary's hyphenation should be followed even when the compound adjective precedes a noun. Hyphens are unnecessary in other unambiguous, regularly used compound adjectives.
- Man-eating shark (as opposed to man eating shark, which could be interpreted as a man eating the meat of a shark)
- Wild-goose chase (as opposed to wild goose chase, which could be interpreted as a goose chase that is wild)
- Long-term contract (as opposed to long term contract, which could be interpreted as a long contract about a term)
- Zero-liability protection (as opposed to zero liability protection, which could be interpreted as there being no liability protection)
It may be appropriate to distinguish between compound modifiers whose adverb has the suffix -ly, such as quickly and badly, and those whose adverb does not, such as well. The -ly suffix on an adverb allows readers to understand its lexical category (if not in the technical sense, then at least in the sense of the intended meaning), showing that it is intended to modify the adjective that it precedes and so not requiring hyphenation. Quickly and badly are unambiguously adverbs. Other adverbs (such as well) can commonly be used as adjectives; therefore these adverbs without the -ly suffix are accompanied by a hyphen. For example, one could speak of a well-known actress or a little-known actress.
Furthermore, the word very in a compound modifier is generally not accompanied by a hyphen. Where both (or all) of the words in a compound modifier are nouns, it is seen as not necessary to hyphenate them, as misunderstanding is unlikely.
If the compound modifier that would otherwise be hyphenated is changed to a post-modifier—one which is located after the modified noun phrase—then the hyphen is conventionally not necessary: the actress is well known.
See also 
- VandenBos, Gary R., ed. (2010). Publication Manual of the American Psychological Association (6th ed.). American Psychological Association. section 4.13. ISBN 1-4338-0559-6. "Hyphenation. Compound words take many forms. [...] The dictionary is an excellent guide for such decisions. [...] When a compound can be found in the dictionary, its usage is established and it is known as a permanent compound."
- The Chicago Manual of Style (16th ed.). University of Chicago Press. 2010. section 7.85. ISBN 978-0-226-10420-1. "In general, Chicago prefers a spare hyphenation style: if no suitable example or analogy can be found either in this section or in the dictionary, hyphenate only if doing so will aid readability"
- Merriam-Webster's Manual for Writers and Editors. Merriam Webster. 1998. p. 73. ISBN 978-0-87779-622-0. "Permanent compound adjectives are usually written as they appear in the dictionary even when they follow the noun they modify"
- The Chicago Manual of Style (16th ed.). University of Chicago Press. 2010. section 7.80. ISBN 978-0-226-10420-1. "Where no ambiguity could result, as in public welfare administration or graduate student housing, hyphenation is unnecessary"
- Hyphens – Punctuation Rules, GrammarBook.com
- Guardian and Observer Style Guide (see section on adverbs)
- "Spelling and Hyphenation". Northeastern University Guidelines. Northeastern University. Retrieved March 25, 2011.
- Compound Modifiers, DailyWritingTips.com
- Compound Modifiers, Writing.com
- The Chicago Manual of Style, 15th ed. 2003, Clause 5.92, p. 171
- The Associated Press Stylebook and Libel Manual, Addison-Wesley Publishing Company, Inc. (1992)
Further reading 
- "Hyphens" in the Style Guide of the Economist