Hyphens in Prefixes

The Quick Answer
A prefix is a half word (e.g., anti-, ex-, post-, pre-) placed before a word to modify its meaning. Most prefixed words can be written with or without a hyphen after the prefix.

As a guideline, avoid the hyphen if you can, but if the unhyphenated version is a spelling mistake (let your spellchecker do its job) or looks too unwieldy for you, use a hyphen.

That said, you should always use a hyphen with ex- and self-, and you should never allow ambiguity to creep in (e.g., you must write re-press the shirt not repress the shirt). There are more-detailed guidelines below.

When Do You Need a Hyphen with a Prefix?

There is often confusion over whether a hyphen should be used with a prefix. In other words, should you write re-consider or reconsider, or anti-aircraft or antiaircraft?

Unfortunately, there is no simple rule governing this, but there are some guidelines.

Guiding Principles for Hyphens with Prefixes

If it's not a spelling mistake to avoid the hyphen and you can bear how the word looks without it, then avoid the hyphen.

Often, it's your choice whether to use a hyphen. Lots of prefixed words can be written with or without a hyphen. The underlying guideline is:

Try to avoid a hyphen with a prefix. However, if you feel the word looks too unwieldy without a hyphen or if your spellchecker highlights it as wrong, then add a hyphen.
  • Antiaircraft
  • (Antiaircraft is not wrong, but it looks a little unwieldy. If you feel the same way, go for anti-aircraft.)
The following words are all correct with or without a hyphen. Remember though, the guiding principle is avoid the hyphen if you can.
  • Proactive
  • Prehistoric
  • Ultraviolet

Use a Hyphen with a Proper Noun

If your prefix sits before a proper noun, you should use a hyphen. Examples:
  • un-British
  • pro-Nazi

Do Not Allow the Same Vowel to Double Up

If the prefix ends in the same vowel that the root words starts with, separate them with a hyphen. Examples:
  • semi-industrious
  • re-enter
  • ultra-argumentative
However, particularly when the vowel is an o, if you can bear how the word looks without a hyphen and your spellchecker lets it through, then omit the hyphen.
  • coordinate
  • cooperate
  • coowner
  • (Your spellchecker (or dictionary) will not let you have coowner.)

You Can Let Different Vowels Double Up

When a prefix ends in a vowel and the root word starts with a different vowel, it is usual to omit the hyphen. Examples:
  • proactive
  • reactivate
  • semiautonomous
  • (However, if your spellchecker doesn't like it or you cannot bear how it looks, go for a hyphen. For many, semiautonomous looks too unwieldy.)

Use a Hyphen with Ex and Self

The prefixes ex and self are usually followed by a hyphen. Examples:
  • ex-husband
  • self-aware

Eliminate Ambiguity Every Time

If the unhyphenated version could be confused with a different word, add the hyphen. (This is most common with the prefix re. Examples:
  • re-cover / recover
  • (If there were no hyphen in re-cover, it could be confused with recover, meaning return to a normal state.)
  • re-press / repress
  • (If there were no hyphen in re-press,Could be confused with repress, meaning subdue with force.)

Enjoy the Leniency

Most prefixed words exist in both forms. As you might have noticed in the guidelines above, it is often down to how the writer feels about the word.
  • The attack would take place at night as the anti-government troops did not possess infrared goggles.
  • (In this example, the writer did not like the look of antigovernment or infra-red, so chose the versions above. That's fine. Readability – as the writer sees it – trumps all guidelines.)

See Also

Alternatives to hyphens in compound adjectives Hyphens in compound adjectives Hyphens in compound nouns What are affixes? What are suffixes?