A compound adjective is basically an adjective phrase made up of two or more words that serve as a single adjective when used together. Much like other types of adjectives, such a phrase is commonly used within a sentence to describe a noun, and it usually comes just before the word it describes. Depending on the words that make up the phrase, hyphenation may be required for the sake of clarity in meaning, though proper nouns do not usually require this. A compound adjective can be seen in a sentence like “A well-known actor entered the room,” in which “well-known” is hyphenated to indicate a single adjective phrase.
The two basic indicators of a compound adjective are the presence of two or more words together and the fact that it still acts as one single adjectival idea. A simple example of this type of phrase can be found in a sentence like “The four-year-old boy picked up the ball,” in which “four-year-old” is a single phrase that acts as an adjective describing “boy.” Some phrases can consist of two or more words and not be a compound adjective, such as the use of an adverb that describes the adjective itself. A sentence like “The large, orange cat sat down,” includes two or more words together, but “large” and “orange” act as two separate adjectives rather than working together to form one.
This idea, that the words work together to form a single idea, is an important aspect of a compound adjective. In something like “The four-year plan,” the phrase “four-year” describes the plan in a single way, expressing the nature of the plan. This is similar to other compound adjectives such as “long-term” or “full-time” that express a comparable idea. In contrast to this, a statement like “My big, old dog,” includes two separate adjectives, “big” and “old,” that describe different aspects of the dog.
Hyphenation of a compound adjective is not always required, though it is quite common for many words. If a phrase includes two nouns, two adjectives, or a similar structure, then hyphenation is typically used to avoid confusion. When a proper noun or other type of title is used that requires capitalization, further separation is not usually necessary; a phrase like “The Gone with the Wind tickets had sold out,” clearly indicates the capitalized title describes “tickets.” Italicization can also be used, like in that example, for writing formats in which titles are italicized or for indication of a compound adjective from a different language.