Hyphenation generally depends on the word’s or words’ role in a sentence. There is not a hard-and-fast rule for the hyphenation of compound words (e.g. anti-slavery/antislavery; socio-economic/socioeconomic), but whatever form is adopted should be used consistently within a single document. Do not use an en rule in place of a hyphen.
Hyphens tend to clutter up text (particularly when the computer breaks already hyphenated words at the end of lines). Inventions, ideas and new concepts often begin life as two words, then become hyphenated, before finally becoming accepted as one word: "Wire-less" and "down-stairs" were once hyphenated. Prefixes such as macro, micro, mega, mini, multi, over, super and under rarely need hyphens.
There is no need to use hyphens with most compound adjectives, where the meaning is clear and unambiguous without: civil rights movement, financial services sector, work inspection powers etc. Hyphens should, however, be used to form short compound adjectives, eg two-tonne vessel, stand-up comedian, three-year deal, 19th-century artist, etc.
Also use hyphens where not using one would be ambiguous, eg to distinguish "black-cab drivers come under attack" from "black cab-drivers come under attack".
Do not use hyphens after adverbs ending in -ly, eg politically naive, wholly owned, but when an adverb is also an adjective (eg hard), the hyphen is required to avoid ambiguity- it's not a hard, pressed person, but a hard-pressed one; an ill-prepared report, rather than an ill, prepared one.<ref>http://www.guardian.co.uk/styleguide/page/0,,184838,00.html Guardian Style Guide</ref>
Compound modifiers that follow the noun are not hyphenated:
- the book is up to date.
- the building is from the twentieth century.
The hyphen is used if the modifier precedes the noun:
- the up-to-date book.
- the twentieth-century building.
Compounds that contain an adjective should be hyphenated when they precede the noun:
- first-rate research; low-level energy.
Compound nouns need not be hyphenated:
- labour market research; hospital bed statistics.
Compounds made up of an adjective and a verb participle should always be hyphenated:
- good-looking; better-rated.
Adverbs ending in ‘-ly’ are not hyphenated:
- frankly discussed subject; newly discovered fact.
Hyphenate words to avoid confusion or mispronunciations:
- pre-eminent; anti-intellectual; de-emphasise.
Use hyphens in spelled out numbers, in compass points and in fractions:
- twenty-one; fifty-five; south-west; north-east; two-thirds.