Use of phrasal adjectives, combinations of two or more words that as a grammatical unit provide additional details to nouns, is complicated by standards of usage pertaining to hyphens. This post discusses various types of phrasal adjectives (some of which have, since they were coined, been fused to form single words).
Just as there are three structural types of compound nouns (closed, as with hindsight, hyphenated, as with life-form, and open, as with “cell phone”), adjectives are also built in one of (the same) three ways.
Occasionally, when a phrasal adjective is formed, its original hyphenated form is superseded by a closed treatment, as with the simple adjective heartwarming, which started out as the phrasal adjective “heart warming.” How does one learn which phrasal adjectives have been fused? Consult a dictionary.
Hyphenated phrasal adjectives are further subdivided into two categories: temporary and permanent. Temporary phrasal adjectives are phrases consisting of an adjective and another form of speech, such as the adverb-adjective team of “well trained.” Note, however, that this instance of this phrasal adjective, though located in a discussion about hyphenation of such parts of speech, is not hyphenated. Why? Because most phrasal adjectives, those designated as temporary, are hyphenated only before a noun: “That is a well-trained dog,” but “That dog is well trained.”
Some phrasal adjectives are considered permanent; one example is “short-lived,” and though it should technically be hyphenated after a noun (as in “Their triumph was short-lived”), The Chicago Manual of Style recommends forgoing hyphenation in most such cases when no misreading is likely.
How is a writer to know which category a phrasal adjective belongs to? Consult a dictionary. Note, however, that some terms are not obvious. On Merriam-Webster’s website, “cutting edge” is designated as a noun, and the adjectival form appears as a footnote to the noun’s definition. But there it is: “cutting-edge.” That phrasal adjective, with a hyphen, is permanent—it’s in the dictionary—so hyphenate it before a noun, but again, per Chicago, it can be left open after the noun.
One form of phrasal adjective that usually needs no hyphenation is one beginning with an adverb ending in -ly, such as in “happily married couple.” But if the phrasal adjective is extended, as in “not-so-happily-married couple,” hyphenate it.
Finally, there’s the permanent open phrasal adjective, such as “information technology,” which never needs hyphenation (unless, again, it is extended, as in “information-technology-related discussion”). Why? Repeat after me: “It’s in the dictionary.” Note, too, that hyphenation is omitted in proper names used as phrasal adjectives, as in “Los Angeles freeways” and “the Sylvester Stallone school of acting,” though, again, if another word is included in the phrasal adjective, an exception is made. However, in this case, because two or more words constitute a single concept, an en dash used as a super-hyphen should link the proper name to the additional word: “Los Angeles–style traffic” and “Sylvester Stallone–lite acting.”
Another exception is for terms of art, or jargon that a particular publication or industry deems so well established that, in a specific context, no confusion is likely; therefore, “open source,” which is treated as a hyphenated permanent phrasal adjective in the dictionary, is often left open before or after a noun in technology-related content. (In general-circulation publications, however, it is generally hyphenated before a noun.)
A final caution: Various phrasal adjectives beginning with a common element are not necessary styled the same: A student in the first grade is a first-grade student, but firsthand is a closed compound, and “first person,” though treated as an open compound in the dictionary (like “cutting edge,” mentioned above), is listed as a noun, and when employed as a phrasal adjective, it should still be hyphenated before a noun.