hyphens: prefixes

Follow these guidelines when deciding whether to use a hyphen after a prefix.

Ex, self, all and quasi

Hyphenate nouns or adjectives beginning with the prefixes ex (meaning “former”), self, all and quasi:

• ex-wife
• ex-premier Getty
• self-control
• self-assured
• all-inclusive
• all-powerful
• quasi-judicial
• quasi-stellar

However, when self is the base word to which a suffix is added, do not hyphenate:

• selfish
• selfhood
• selfsame
• selfless

SI/metric prefixes

Write SI/metric unit compounds as one word:

• centimetre
• gigagram
• kilokelvins
• milliampere

Other common prefixes

Most words beginning with the following prefixes are written as one word: after, ante, anti, bi, co, counter, de, down, extra, infra, inter, intra, iso, macro, micro, multi, over, photo, poly, post, pre, pro, pseudo, re, retro, semi, stereo, sub, super, trans, tri, ultra, un, under and up.

• afterthought
• antecedent
• antiballistic
• bimonthly
• covalent
• counterclockwise
• decertify
• downturn
• extrasensory
• infrastructure
• interstellar
• intramural
• isometric
• macrocosm
• microscope
• multistage
• overestimate
• photovoltaic
• polyurethane
• postnatal
• preposition
• proconsul
• pseudonym
• readapt
• retroactive
• semiquaver
• stereophonic
• subspecies
• supernatural
• transcontinental
• triennial
• ultrasound
• unassuming
• underrate
• upswing
• upwind

However, there are many exceptions. Check the Canadian Oxford Dictionary when in doubt, and see below for three specific types of exceptions.

Hyphenate for clarity

Use a hyphen when the word following the prefix begins with the same vowel as the one with which the prefix ends or when the compound’s appearance would be confusing without the hyphen:

• co-opt
• pre-eminent
• re-educate
• semi-invalid
• co-author
• de-icing

Hyphenate to preserve a difference in meaning

In certain cases, use the hyphen to preserve a difference in meaning between the hyphenated and the solid compound:

• re-cover (cover again) vs. recover (get better, get back)
• re-create (create again) vs. recreate (take recreation)
• re-solve (solve again) vs. resolve (settle)
• re-sign (sign again) vs. resign (quit a job)

Hyphenate before proper nouns and adjectives

Hyphenate a prefix joined to a proper noun or adjective:

• mid-July
• sub-Arctic
• neo-Christian
• trans-Siberian
• pro-Canadian
• un-American

Exceptions: transatlantic, transpacific

Chemical terms

Hyphenate chemical terms preceded by an italicized prefix:

• cis-dimethylethylene
• ß-lactose