Compound of two or more words that collectively modify a noun
A compound modifier (also called a compound adjective, phrasal adjective, or adjectival phrase) is a compound of two or more attributive words: that is, two or more words that collectively 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 constituents of compound modifiers need not be adjectives; combinations of nouns, determiners, and other parts of speech are also common. For example, "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 are typically not (if they are temporary compounds), unless they are permanent compounds attested as dictionary headwords.
Compound adjectives.
Words that function as compound adjectives may modify a noun or a noun phrase. Take the English examples "heavy metal detector" and "heavy-metal detector". The former example contains only the bare adjective "heavy" to describe a device that is properly written as metal detector; the latter example contains the phrase "heavy-metal", which is a compound noun that is ordinarily rendered as heavy metal absent an accompanying adjective. In the latter example, however, "heavy-metal" functions as a compound adjective that modifies the noun "detector".
Whether a word sequence such as "heavy + metal + detector" implies a "compound adjective + noun" or "bare adjective + compound noun" depends on the punctuation. For instance, "heavy-metal detector" and "heavy metal detector" can refer to quite different things: "heavy-metal detector" implies a device that detects heavy metals (wherein "heavy-metal" functions as a compound adjective that modifies the noun "detector"). By contrast, "heavy metal detector", without the hyphen, refers to a metal detector that is heavy. "Heavy" is a bare adjective that modifies the compound noun "metal detector". Thus, "heavy metal detector" is a "bare adjective + compound noun" sequence.
A strategy to avoid conflation of "compound adjective + noun" versus "bare adjective + compound noun" sequences is to clearly distinguish the usage of an attributive adjective and a noun adjunct. Accordingly, the phrase "heavy metallic detector" unequivocally employs a compound adjective to describe a "weighty detector made of metal."
Hyphenation of elements in English.
Conventionally, and with the support of modern writing guides, compound modifiers that appear "before" a noun phrase should 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. Hyphens join the relevant words into a single idea, a compound adjective.
A compound modifier that is spaced rather than hyphenated is referred to as an open compound.
When a numeral and a noun are used in a compound modifier that precedes a noun, the noun takes the singular form. For example, the 2021 Belmont Stakes was a "12-furlong race" and an "eight-horse race", not a "12-furlongs race" or an "eight-horses race".
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 follows 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.
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; these adverbs without the "-ly" suffix therefore commonly take a hyphen. For example, one could speak of a "well-known actress" or a "little-known actress". 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".
Finally, 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.
Examples.
Examples using an en dash for attributive compounds.
An en dash may be used for attributive compounds to enhance readability and eliminate ambiguity.
Other languages.
Hungarian.
Hungarian orthography describes three types of this modification in spelling, as described in the main article.
Japanese.
Japanese 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 (noun + "i"-adjective) and ("i"-adjective stem + "i"-adjective).
References.
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