A prefix is a half word (e.g., anti-, ex-, post-, pre-) added to the front of a word to modify its meaning.

Prefixes (added to the front of words) contrast with suffixes (added to the back). Prefixes and suffixes are known as affixes.

Easy Examples of Prefixes

  • microscope
  • tripod
  • devalue
  • re-establish
  • (Sometimes, a prefix is written with a hyphen. More on this to come.)

Real-Life Examples of Prefixes

The four most common prefixes are dis-, in-, re- and un-. (These account for over 95% of prefixed words.) Here they are in some short quotations.
  • To make people disappear, ask them to keep their promises. (American academic Mason Cooley)
  • It's inconsiderate to expect someone who isn't your boss or your sister-in-law to know who you are. (American author Letitia Baldrige)
  • Next time I see you, remind me not to talk to you. (Comedian Groucho Marx)
  • You can't die with an unfinished book. (Author Terry Pratchett)
There is a list of common prefixes in the entry on affixes.

Why Should I Care about Prefixes?

By far the most common question with prefixes is whether to use a hyphen or not. In other words, should you write re-consider or reconsider, or anti-aircraft or antiaircraft?

As a general guideline, avoid a hyphen, but if the unhyphenated version is a spelling mistake (let your spellchecker do its job) or looks too unwieldy for you, then use a hyphen.
  • proactive, prehistoric, ultraviolet
  • (These words can be written with or without a hyphen, i.e., the hyphenated versions are not spelling mistakes. Remember though, the guiding principle is to avoid the hyphen if your spellchecker lets you.)
  • co-opt
  • (Your spellchecker will bleat at coopt, so you must use a hyphen.)
  • Antiaircraft
  • (Antiaircraft is not wrong, but it looks a little unwieldy. If you feel the same way, go for anti-aircraft.)
This overarching guidance will see you through most situations. Below are six points offering a little more detail:

(1) Use a hyphen with a proper noun

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

(2) Do not allow the same vowel to double up (unless it's an o)

If the prefix ends in the same vowel that the root words starts with, separate them with a hyphen.
  • 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 [wrong]
  • (Your spellchecker, or dictionary, will not allow coowner.)

(3) 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.
  • proactive, reactivate, semiautonomous
  • (Remember, if your spellchecker doesn't like it or you cannot bear how it looks, go for a hyphen. For many, semiautonomous looks too unwieldy.)

(4) Use a hyphen with ex and self

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

(5) 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.)
  • 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, it could be confused with repress, meaning subdue with force.)

(6) Enjoy the leniency

Most prefixed words exist in both forms. As you might have noticed from the guidelines above, it is often down to how the writer feels about the word.
  • The attack was at night because the anti-government troops did not possess infrared goggles.
  • (Here, the writer did not like the look of anti-government or infra-red, so chose the versions above. That's fine. Enjoy the leniency.)
  • Why is non-hyphenated hyphenated? The irony!
  • (This is a popular internet meme. In fact, nonhyphenated doesn't need a hyphen, but the joke fails without it. Enjoy the leniency.)
In the entry on affixes, we talk about how you can use your understanding of prefixes to:

(1) Reduce your word count (e.g., not wanted > unwanted, think again > rethink).

(2) Improve your spelling by breaking a word down into prefixes and suffixes and tackling the parts one at a time (e.g., dis-respect-ful-ly).

(3) Decipher the meaning of a long word (e.g., im-practical = not practical). (A quick aside: Why does inflammable mean flammable? That's crazy.)

Key Point

  • Don't use a hyphen with a prefix, but if your unhyphenated word is a spelling mistake or you can't bear how it looks, then use a hyphen.
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