Compound modifier

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 combination with other modifiers. In the preceding sentence, "single-word" is itself a compound modifier.

The constituent words of compound modifiers need not be adjectives; combinations of nouns, determiners, and other parts of speech are also common: man-eating (shark) and one-way (street).

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 compounds used as predicates will typically not be (if they are temporary compounds), unless they are permanent compounds attested as dictionary headwords.

Compound adjectives, like normal adjectives, modify noun phrases. In the phrases heavy metal detector and heavy-metal detector, the latter is a compound adjective because the modifier is made of two words used in combination.

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. Heavy metal detector and heavy-metal detector refer to subtly different things: in the first, heavy modifies metal detector, to describe a heavy device that detects metals, while in the second example, heavy modifies metal which together modify detector, to describe a device that detects heavy metals.

Conventionally, and with the support of modern writing guides, compound modifiers that appear before a noun phrase must include a hyphen between each word, subject to certain exceptions. Hyphens are used in this way to 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, a compound adjective.

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