Hyphens glue words together so the reader knows they are linked.

Some of the rules for using hyphens are straightforward, but there are several exceptions and often you have to decide whether a hyphen is necessary. That is why some people dislike hyphens and probably why the Oxford University Press style manual says:

If you take hyphens seriously, you will surely go mad.

Single concept
One of the most common uses of a hyphen is to link two or more words that modify the following noun. The hyphenated words act as a single concept and are sometimes called a compound adjective.

full-time workers

You don’t usually need a hyphen when the descriptive concept comes after the noun.

the employee works full time

If a phrase consists of an adverb, adjective and noun, it is usually not hyphenated because the adverb is modifying the adjective rather than the words acting as a single concept.

a very good meal (adverb, adjective, noun)

However, to complicate matters, hyphens are used with some adverbs including better, best, ill, least, little, most, much, worse, worst and well if they are followed by the past tense of the verb.

well-liked executive
well-known fact

New words
Hyphens are often used when new words are invented out of existing words. Many of these hyphens drop out over time.

web site, web-site, website

Commonly used expressions
Hyphens creep into some commonly used expressions.

state-of-the art
mother-in-law

Hyphens with numbers
Hyphenate numbers, fractions and ages.

thirty-four
three-quarters
five-year-old child

Hyphens with single letters
Hyphens are often used when a single letter is added to the front of a word to make a new word, though many e-words now drop the hyphen.

X-ray
e-mail (now usually email)

Hyphens and prefixes
Use a hyphen:

  • If the word that follows the prefix starts with the same vowel, though some double vowel usages have now become accepted

de-emphasise, but coordinate

  • For the prefixes quasi-, all-, ex– and self-

self-effacing

  • To join a prefix to a proper noun

pro-American

  • Between a prefix and a date

pre-1950

  • To avoid ambiguity

re-form and reform

Hyphens and suffixes
Hyphens are generally not used with suffixes (threefold), but you need them occasionally for clarity. Hyphens are often used with the suffixes –elect, -like and -wise.

President-elect
money-wise