Hyphenate compound adjectives which precede nouns. If two compound adjectives end in the same word and modify the same noun, the last word should be omitted from the first compound adjective, as below.
- The state-of-the-art model
- A long-term relationship
- A 20-year-old boy
- 19th- and 20th-century literature
A hyphen is generally unnecessary when the compound adjective follows the noun.
- Their relationship is long term.
- The boy is 20 years old.
However, some adjectives are always hyphenated.
- The model was state-of-the-art.
Do not hyphenate compound adjectives which begin with the word very or with an adverb ending in -ly. If these words are not the first word of the adjective, the adjective should usually be hyphenated.
- Correct: The very tired man slept in his wonderfully soft bed.
- She received a not-very-nice phone call from her boss.
- Incorrect: The very-tired man slept in his wonderfully-soft bed.
- She received a not very nice phone call from her boss.
Use hyphens as needed to clarify the meanings of compounds. Also use hyphens in words which may be misread without a hyphen.
- Small-business owner (owner of a small business) vs. small business owner (small owner of a business)
- Re-creation (creation occurring again) vs. recreation (activity carried out for fun)
Compounds which are not adjectives do not follow any simple rule. In most cases, if you are not sure whether a compound should be hyphenated, consult a dictionary or Google Ngrams to determine the most common form. The most common non-adjective compounds in synopses are compound age nouns, which are hyphenated (e.g. an 18-year-old).