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.
Contents
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
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.
Hyphenation of elements
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.
Exceptions
Major style guides advise consulting a dictionary to determine whether a compound adjective should be hyphenated; compounds entered as dictionary headwords are permanent compounds, and for these, the dictionary's hyphenation should be followed even when the compound adjective precedes a noun. According to some guides, hyphens are unnecessary in familiar compounds used as adjectives "where no ambiguity could result", while other guides suggest using hyphens "generally" in such compounds used as adjectives before nouns.
Examples
Exceptions
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.
Japanese
Japanese equivalents of adjectives can compound. This is quite common for na-adjectives, which function essentially as attributive noun phrases, while it is relatively uncommon for i-adjectives, and is much less common than Japanese compound verbs. Common examples include omo-shiro-i (面白い, interesting) "face-whitening" (noun + i-adjective) and zuru-gashiko-i (狡賢い, sly) "crafty-clever" (i-adjective stem + i-adjective).