Use hyphens to clarify meaning by joining together compound adjectives when they come before a noun but not after the noun.
Des Moines is considered a medium-sized city.
The 20-year-old pitcher thought he was too young for the big leagues.
The business asked for a permit to operate on government-owned land. Compare with: The business asked for a permit to operation on land that is government owned.
He refuses to let his children watch R-rated movies.
Note that with adverbs used with adjectives, a hyphen can change the meaning of a sentence:
The couple asked to see more unusual houses.
(The couple wants to see more houses that are unusual.)
The couple asked to see more-unusual houses.
(The couple wants to see houses that are more unusual than
those they've seen.)
Some common compound adjectives don't require a hyphen:
high school student, real estate agent, human rights policy
Use hyphens for compound numbers and fractions.
It takes a two-thirds vote of both houses of Congress to override a
The glass is three-quarters full.
Forty-six is written 46 in Arabic numerals and XLVI in Roman numerals.
Certain prefixes take hyphens:
New England advocated a pro-British policy; the South was pro-French.
Always check with your dictionary to see which prefixes take a hyphen and which don't.
Note that adverbs ending in -LY don't take a hyphen if they are part of a compound adjective.
She opened the door and admitted the nattily attired gentleman.
Many compound nouns take hyphens -- but only your dictionary can tell you which ones do and which don't (and different dictionaries don't always agree):