Updated April 21, 2026
Spotlight on CMOS 7.96, section 4
A prefix is a partial word that joins to the front of another word (and sometimes a phrase) to create a new word with a different meaning. The pre- in prefix is a prefix, for example.
More often than not, prefixes combine to form single, unhyphenated words, as in antidepressant, cooperation, infrastructure, nonexistent, and postdoctoral. But sometimes a hyphen is called for, either to avoid an awkward appearance or to prevent a possible misreading: anti-inflammatory, de-emphasize, re-cover (as opposed to recover), and un-American.
That second category is where things can get a little fuzzy, but CMOS can help.
Spell-Check Won’t Save You
Writers and editors can’t afford to look up every single word they read; that would take too long. Nor, for hyphenation, can we count on our word processors to save us.
For example, there are about 140 examples of compounds formed with prefixes in section four of Chicago’s hyphenation guide. Microsoft Word’s spelling checker, with the proofing language set to “English (United States),” currently accepts about three-fourths of these by default in both their hyphenated and unhyphenated forms, or about two-thirds when the grammar checker is included.
So if a document includes both antihero and anti-hero and megavitamin and mega-vitamin and nonevent and non-event, Word won’t flag any of those as errors, leaving it up to you to decide (and to keep track of your choices once you do).*
Again, most of the time a prefix can combine with another word without a hyphen. So if a word looks right without a hyphen (i.e., closed up), and your word processor doesn’t object, then you can usually move on. It’s those fuzzy cases, where a hyphen seems like it might be helpful, that cause the most trouble. You can always check a dictionary (or ask AI), but what if you don’t like what you find there?
General Principles
Fortunately, there are some general principles for deciding when to hyphenate prefixes. Here’s a summary of the advice in CMOS:
Compounds formed with prefixes are usually closed except
- before a capitalized word or a numeral (un-American, pre-1950);
- before a compound term (non-self-sustaining, pre–Vietnam War [see also CMOS 6.86]);
- to separate two i’s, two a’s, and other combinations of letters or syllables that might cause misreading (anti-intellectual, extra-alkaline, pre-authorize);
- to separate the repeated terms in a double prefix (sub-subentry); or
- when a prefix stands alone (over- and underused).
Rules 1, 2, 4, and 5 are mostly straightforward and easy to apply.
Rule no. 3 is the one you’ll need for the fuzzier cases. Double i’s and a’s are always hyphenated. But beyond those two combinations, this rule gives you permission to intervene whenever your editorial sense kicks in and says, This doesn’t look right without a hyphen.
Trust Your Instincts, but Check a Dictionary (or Two)
Most readers would agree that antiintellectual and intraarterial need hyphens. But what about two e’s? Merriam-Webster lists reenact and reenter but de-emphasize and de-escalate; some writers, however, would prefer to hyphenate all four of those terms.
If that’s you, get a second opinion. For example, re-enact and re-enter (with hyphens) are listed as main entries in Google’s dictionary (search “define reenact” and “define reenter”); reenact and reenter (no hyphens) are included as variants. Google relies on Oxford Languages for its definitions, so it seems likely that its hyphenation choices reflect a mix of American and British usage, among other varieties of English. The spellings in M-W, by contrast, generally reflect American usage.
We would be remiss if we didn’t note that words formed with prefixes are more likely to be hyphenated in British English than in American English. For example, nonevent, the usual US spelling, becomes non-event in the UK, and British usage also calls for re-enact and re-enter. Writers and editors working with British English can consult the Oxford English Dictionary or the British spellings in Collins, among other resources.
But even if you’re working with American English, you can look to the UK to justify your desire for the occasional hyphen not found in CMOS or M-W. In a lot of cases, hyphens make words more readable, not less. No one would object, for example, to re-enter rather than reenter, consistently applied.
So if you prefer re-enact and re-enter (and you’re in a position to decide), add these forms to your style sheet and move on.
More Examples
Let’s look at a few more examples featuring terms that are likely to cause a bit of trouble. Some of these have come to us in recent queries to our Q&A; others are simply tricky.
antiracist/anti-racist. This term could go either way, and usage varies. Chicago continues to show it as one word, but M-W lists it with a hyphen.† Choose one and be consistent.
coauthor/co-author. Chicago prefers coauthor—and coeditor and coworker—which reflect M-W’s current entries for all three. But M-W also lists co-author, co-editor, and co-worker (as equal variants), and the hyphens are preferred in British usage. So if the unhyphenated versions bother you (perhaps you see a cow in coworker), add hyphens (and be consistent).
counterrevolutionary/counter-revolutionary. Prefixes that result in a doubled consonant (as the rr in counterrevolutionary) are almost always closed in Chicago style. But if a term seems unclear as one word, or if you’re writing for a non-US audience, a hyphen may be appropriate. Check your dictionary (or dictionaries).
neoorthodox/neo-orthodox. CMOS lists neoorthodox as an example, in line with M-W. But dictionaries disagree on this one; for example, the entry in American Heritage has a hyphen, as does the entry in the OED. So if the double o bothers you, hyphenation is an option.
nonnegotiable/non-negotiable. See counterrevolutionary, above. Unless you’re following UK spelling, this one is (usually) nonnegotiable.
nonissue/non-issue. This is another one where CMOS and M-W agree, so it’s a nonissue—except in British usage (naturally).
prochoice/pro-choice. Dictionaries everywhere (including M-W) seem to agree that pro-choice (and pro-life) should be hyphenated. And those terms are more readable that way, so count these as two more exceptions to Chicago’s usual rule.
reedit/re-edit. CMOS and M-W agree on this one: reedit. But as with reenact and reenter, a hyphen can be helpful, as British usage reminds us. Follow your editorial instincts, house style permitting.
* * *
We could go on, but it should be clear by now that in addition to straightforward cases like pre-1950 and non-US, Chicago reserves hyphens for compounds that might be awkward without them. It should also be clear that this is a judgment call that will depend in part on your preferred dictionary—and on your variety of English.
Whatever you decide for any one term, don’t forget the second rule of editing: Be consistent.‡
* Furthermore, in our tests, Word’s grammar checker flagged certain hyphenated terms but didn’t always suggest the expected fix. For example, Word flagged both mini-van and proto-galaxy and suggested mini van and proto galaxy (each as two words) rather than the established compounds minivan and protogalaxy.
† As late as 2017, Merriam-Webster.com listed antiracist, without a hyphen, under its entry for the prefix anti-. M-W now lists it as a separate entry, where it’s hyphenated. Many other anti- terms have followed this same progression in M-W, from anti-academic to anti-modern to anti-woman.
‡ The first rule: Do no harm.
Typewriter image by WINDCOLORS / Adobe Stock. Text altered for this post.
For more on prefixes, take our quiz.
Please see our commenting policy.
