General. Compounds may be written as two or more separate words, with hyphen(s), or as a single word. There is a tendency for compounds to develop into single words when they come to be used more frequently: data base, data-base, database.
Use hyphens sparingly but to good purpose: in the phrase crude oil production statistics a hyphen can tell the reader that ‘crude’ applies to the oil rather than the statistics.
Sometimes hyphens are absolutely necessary to clarify the sense:
- re-cover — recover; re-creation — recreation; re-form — reform; re-count — recount
The following are examples of well-used hyphens:
- user-friendly software; two-day meeting; four-month stay (but four months’ holiday); tonne-kilometre; person-day
In adverb-adjective modifiers, there is no hyphen when the adverb ends in -ly:
- occupationally exposed worker; a beautifully phrased sentence
With other adverbs, however, a hyphen is usually required:
- well-known problem; above-mentioned report; hot-rolled strip (but a hotly disputed election); broad-based programme (but a broadly based programme)
An adjective formed out of a noun and a participle should be hyphenated:
- drug-related crime, crime-fighting unit; oil-bearing rock
Many phrases are treated as compounds, and thus need a hyphen, only when used as modifiers:
- policy for the long term, but long-term effects
- production on a large scale, but large-scale redundancies
- balance of payments, but balance-of-payments
- policy cost of living, but cost-of-living
- index loans with low interest, but low-interest loans
- measures for flood control, but flood-control measures
Chemical terms. Note that open compounds designating chemical substances do not take a hyphen in attributive position: boric acid solution, sodium chloride powder.
Prefixes are usually hyphenated in recent or ad hoc coinages:
- anti-smoking campaign, co-responsibility levies, co-sponsor, ex-army, non-resident, non-flammable, pre-school, quasi-autonomous
If they are of Latin or Greek origin, however, they tend to drop the hyphen as they become established:
- antibody, codetermination, cooperation, subcommittee, subparagraph
Others are more resistant to losing the hyphen:
- end-user, end-phase, end-product, all-embracing, all-metal, off-market operations, off-duty
- endgame, nonsense, overalls
Nouns from phrasal verbs. These are often hyphenated or written as single words. The situation is fluid: handout, takeover, comeback but follow-up, run-up, spin-off.
Present participles of phrasal verbs. When used as attributes they are generally hyphenated:
- cooling-off period
Avoiding double consonants and vowels. Hyphens are often used to avoid juxtaposing two consonants or two vowels:
- aero-elastic, anti-intellectual, part-time, re-election, re-entry, re-examine
However, the hyphen is often omitted in frequently used words:
- bookkeeping, coeducation, cooperation, coordinate, macroeconomic, microeconomic, radioactive
Numbers and fractions. Numbers take hyphens when they are spelled out. Fractions take hyphens when used attributively, but not when used as nouns:
- twenty-eight, two-thirds completed
- an increase of two thirds
Prefixes before proper names. Prefixes before proper names are hyphenated: pro-American, intra-EU, mid-Atlantic, pan-European, trans-European. Note, however, that transatlantic is written solid.
Coordination of compounds. Hyphenated compounds may be coordinated as follows:
- gamma- and beta-emitters, acid- and heat-resistant, hot- and cold-rolled products
Where compounds are not hyphenated (closed compounds), or should you choose to write them so, they should not be coordinated but written out in full:
- macrostructural and microstructural changes, minicomputers and microcomputers, prenatal and postnatal effects, agricultural inputs and outputs
- macro- and microstructural changes, mini- and microcomputers, pre- and postnatal effects, agricultural in- and outputs
(BUT of course
- macro- and micro-structural changes, pre- and post-natal effects)
Closed compounds in technical texts. Some expressions that are written as separate words in everyday language become closed compounds in more specialist contexts, e.g. pigmeat, longwall. This reflects the fact that in a particular field such expressions have the status of precise terms.
Source: European Commission Directorate-General for Translation
A handbook for authors and translators in the European Commission
Seventh edition: August 2011 Last updated: May 2014