Some compounds are one word (rowboat), some are separate words (hard drive), and some require hyphens (sister-in-law). You should consult a dictionary to be sure. However, the following conventions can help you decide when to use hyphens with compound words.

Hyphenating compound adjectives

Hyphenate most compound adjectives that precede a noun but not those that follow a noun.

In general, the reason for hyphenating most compound adjectives is to facilitate reading.

Without the hyphen, living may seem to modify room dividers.

Commonly used compound adjectives, however, do not usually need to be hyphenated for clarity—income tax reform or first class mail would seldom if ever be misunderstood.

Never hyphenate an -ly adverb and an adjective.

Compound adjectives formed from compound proper nouns are hyphenated if the noun is hyphenated: Austro-Hungarian history but Latin American literature.

Hyphenating coined compounds

You may need hyphens to link coined compounds, combinations of words that you are using in an unexpected way, especially as an adjective.

She gave me her I-told-you-so look before leaving the party.

Hyphenating fractions and compound numbers

Use a hyphen to write out fractions and to spell out compound numbers from twenty-one to ninety-nine, both when they stand alone and when they are part of larger numbers. (Usually such larger numbers should be written as numerals. See Chapter 61.)

Using suspended hyphens

A series of compound words that share the same base word can be shortened by the use of suspended hyphens.

Each student should do the work him- or herself.