What Is a Prefix?

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

Table of Contents

  • Examples of Words with Prefixes
  • The Four Most Common Prefixes
  • A List of Common Prefixes
  • Why Prefixes Are Important
  • Test Time!
What are prefixes?
Prefixes contrast with suffixes, which are added to the back of a word. Prefixes and suffixes are known as affixes.

Examples of Words with Prefixes

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

The Four Most Common 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 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)

A List of Common Prefixes

Here is a list of common prefixes with some examples:
a-, an-withoutamoral, atypical
ante-beforeantecedent, antenatal
co-withco-conspirator, co-pilot
com-, con-withcompanion, contact
de-offdelist, devalue
en-put intoenclose, envelop
ex-out of, formerextract, ex-governor
extra-beyond, more thanextracurricular
homo-samehomonym, homophone
hyper-over, morehyperactive
il-, im-, in-, ir-not, withoutillegal, impractical, inconsiderate, irresponsible
inter-betweeninternet, intersection,
intra-betweenintranet, intravenous
non-not, withoutnonentity, nonstarter,
omni-all, everyomnipresent, omniscient
pre-, pro-before, forwardprecede, project
sub-undersubmarine, substandard
syn-same timesynchronize
super-abovesupervisor, superhuman
tri-threetripod, triceratops
un-notundone, unfinished,
uni-oneunicorn, unilaterally

Why Prefixes Are Important

Here are three noteworthy points related to prefixes.

(Point 1) Are prefixes hyphenated?

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 correct tick, prehistoric correct tick, ultraviolet correct tick
  • (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 correct tick
  • (Your spellchecker will show coopt as an error, so you must use a hyphen.)
  • antiaircraft correct tick, anti-aircraft correct tick
  • (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:

(Rule 1) Use a hyphen with a proper noun.

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

(Rule 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 cross
  • (Your spellchecker, or dictionary, will not allow coowner.)

(Rule 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 that if your spellchecker doesn't like it or you cannot bear how it looks, go for a hyphen. For many, semiautonomous looks too unwieldy.)

(Rule 4) Use a hyphen with 'ex' and 'self.'

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

(Rule 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 to return to a normal state.)
  • re-press / repress
  • (If there were no hyphen in re-press, it could be confused with repress, meaning to subdue with force.)
  • pre-date / predate
  • (If there were no hyphen in pre-date, it could be confused with predate, meaning to prey upon.)

(Rule 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 antigovernment 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.)

(Point 2) Can you add to prefix to reduce your word count?

Often a prefix can be added to reduce your word count by a word. For example:
  • not wanted > unwanted
  • think again > rethink
  • not practical > impractical

(Point 3) Improve your spelling and vocabulary.

In the entry on affixes, we talk about how you can use your understanding of prefixes to:
  • 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).
  • 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

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This page was written by Craig Shrives.