Too many hyphens make text look cluttered, so avoid them where you can, but they are often necessary to avoid ambiguity. See ndash.

Compound adjectives

Use a hyphen to join the parts of compound adjectives, for example:

Green-belt land, zero-gravity environment, state-of-the-art phones, well-behaved patients, ill-considered attack, better-trained staff, a little-known city.

"The boy is six years old" but "a six-year-old boy".

Exception: compound adjectives in which the first word is an adverb ending in "-ly" (e.g. genetically modified food, highly trained staff, doubly special relativity); compound hyphens including a proper name (e.g. the New York cable infrastructure); with foreign expressions (e.g. an ad hoc group).

Measurements

Measurements are a particular form of compound adjective that requires a hyphen to link quantities to their units, as in:

12-centimetre discs, the 3000-kilometre journey

(but not when the measurement is not adjectival, as in: "the discs measure 12 centimetres across"; "the 4-hour flight took us 3000 kilometres"

The same applies when making compound adjectives, such as "16-metre-long tubes", otherwise dubiety creeps in (do we mean 16 tubes each one metre in length or several tubes each 16 metres in length?), but the construction is ugly. Writing around it if possible, as in "the tubes are 16 metres long”, is more elegant.

Exception: sums of money (e.g. a $3 million budget) – see currencies.

Compound nouns

Nouns formed from phrasal verbs are often hyphenated:

"ready for take-off"; "paying for the clean-up" (derived from the phrasal verbs “take off” and “clean up” respectively, which are not hyphenated).

However, beware that some such nouns have become one word, e.g. feedback, breakdown – check this guide if unsure, or the OED if the word you want isn't in this guide.

After certain prefixes

Usually use hyphens after the prefixes self-, non-, ex-, quasi.

Hyphens are not usually needed after other prefixes, such as multi, micro, bi, pre, anti, cyber, sub, inter, co (they should be joined to the rest of the word, with no space), except if both the prefix and the word following begin with a vowel (e.g. pre-eminent, micro-organism); if the combination of letters is awkward or hard to read (e.g. co-worker, sub-item), if the next word begins with a capital letter (e.g. sub-Saharan, inter-American); or to distinguish between another word with a different meaning (e.g. re-sent vs resent). There are exceptions to these rules for words that have become so familiar as to become one word – again, check in this guide or, failing that, the OED.

Ranges (especially dates)

"In the period 1979-82, funding was in short supply", though this often reads better if reworded: "From 1979 to 1982, funding was in short supply".

Don’t use “from 2000-2500 years ago” (or, worse, “between 20-25 ºC”). Write “from 2000 to 2500 years ago” or "2000-2500 years ago", “between 20 and 25 ºC” and so on.

Hyphens are also needed…

When spelling out numbers such as twenty-one, sixty-six, ninety-nine (although house style will dictate using numerals, except at the start of a sentence), and compass points e.g. north-east, south-west.

See section 3 of the full Style Guide for more information about when to use hyphens.