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2.07 Prefixes

(a) Hyphenate a prefix joined to a proper noun or adjective:

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

Exceptions

transatlantic and transpacific.

(b) Hyphenate expressions beginning with the prefixes ex (when it means "former"), self and all, when used to form adjectives or nouns, and those beginning with quasi when used to form adjectives:

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

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

  • selfish
  • selfhood
  • selfsame
  • selfless

(c) 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 Gage Canadian Dictionary when in doubt, and see below and 2.07 Prefixes(a) for specific types of exception.

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

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

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

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

(d) Write SI/metric unit compounds as one word:

  • centimetre
  • gigagram
  • kilokelvins
  • milliampere

(e) Hyphenate chemical terms preceded by an italicized prefix:

  • cis-dimethylethylene
  • ß-lactose