The Stylebook's preferences on whether to use a hyphen following a prefix are based largely on Merriam-Webster. Generally we do not hyphenate. But there are exceptions.
A 2024 change: We no longer generally use a hyphen with these prefixes: out-, post-, pre-, re-. Previous guidance was to use a hyphen with those unless listed separately in the dictionary. This change aligns style on those prefixes both with our overall guidance and with Merriam-Webster.
Another 2024 change: no hyphen in semiautomatic and semiautonomous, adding those to the other no-hyphen semi- constructions.
In addition: While we now generally don't use a hyphen in anti- constructions (a 2024 change), there are a number of exceptions to align with Merriam-Webster.
Three rules are constant:
Use a hyphen if the prefix ends in a vowel and the word that follows begins with the same vowel. Exceptions: cooperate, coordinate, and double-e combinations such as preestablish, preeminent, reenact, reelect.
Use a hyphen if the word that follows is capitalized: un-American, for example.
Use a hyphen to join doubled prefixes: sub-subparagraph.
Here is the style for some commonly used prefixes. (Measurement-related prefixes such as centi- and milli- are listed individually in the book.)
In general, no hyphen with these prefixes except as noted in the above three rules:
a-
ante-
anti- Generally no hyphen is a 2024 change for consistency. But note a number of exceptions. They include: anti-abortion, anti-aggression, anti-apartheid, anti-collision, anti-corruption, anti-cruelty, anti-labor, anti-racist, anti-racketeering, anti-secrecy, anti-terrorism, anti-war.
arch-
after- Generally no hyphen after this prefix when it is used to form a noun: aftertaste, afterlife, aftermarket. Exception: after-party. Follow after- with a hyphen when it is used to form compound modifiers: after-tax results, after-work celebration.
bi-
by- Exception: by-election.
counter-
dis-
down-
ex- No hyphen for words that use ex- in the sense of out of, such as excommunicate. Hyphenate when using ex- in the sense of former, such as ex-president. Do not capitalize ex- when attached to a formal title before a name: ex-President Barack Obama. The prefix modifies the entire term: ex-New York Gov. Mario Cuomo; not New York ex-Gov. Usually former is better.
extra- No hyphen when extra means outside of: extralegal, extraterrestrial, extramarital. Follow extra- with a hyphen when it is part of a compound modifier describing a condition beyond the usual size, extent or degree: extra-base hit, extra-large book.
fore- No hyphen except for these nautical exceptions, based on long-standing practice: fore-topgallant, fore-topsail, fore-topmast.
hydro-
hyper-
in- In general, no hyphen when it means not (inaccurate, insufferable). Other uses without a hyphen: inbound, infighting, indoor, inpatient, infield. A few combinations take a hyphen, however, including in-depth, in-house, in-group, in-law.
infra-
inter-
intra-
mega-
micro-
mid- In addition to the general rules for prefixes, use a hyphen when a figure follows: mid-30s.
mini-
multi-
non-
out- Generally no hyphen is a 2024 change for consistency.
over-
post- Generally no hyphen is a 2024 change for consistency.
pre- Generally no hyphen is a 2024 change for consistency.
re- Generally no hyphen is a 2024 change for consistency. For some words, the sense is the governing factor: recover (regain); re-cover (cover again); recreate (relax); re-create (create again); resign (quit); re-sign (sign again).
semi- This includes no hyphen in semiautomatic and semiautonomous; style on those two words changed in 2024 for consistency.
sub-
super-
supra-
trans-
ultra-
under-
un-
up-
In addition, no hyphen with measurement-related prefixes such as centi- and milli-. See separate entries with more detail on those.
Generally use a hyphen with these prefixes unless listed without a hyphen in Merriam-Webster:
all-
full-
half- Hyphenated combinations include half-baked, half-life, half-truth, half-moon, half-cocked, half-hearted (the latter a 2024 change). Two-word combinations without a hyphen include half dozen, half brother, half off. One word, no hyphen, for some words including halfback, halftone.
Also: halftime as a noun, in keeping with widespread practice in sports copy. But half-time as an adjective outside sports contexts.
like- Follow with a hyphen when used as a prefix meaning similar to: like-minded, like-natured. No hyphen in words that have meanings of their own: likelihood, likewise, likeness.
off- Follow Merriam-Webster. Some examples: offbeat, offhand, offline, offset, offshore, off-brand, off-key, off-limits, off-load.
pan- Most combinations with pan- are proper nouns. Use a hyphen with those, and capitalize both pan- and the proper name it is combined with: Pan-African, Pan-American. No hyphen when combined with a common noun: panchromatic.
part-
pro-
self- Always hyphenate: self-assured, self-government, self-defense
wide- Follow Merriam-Webster. Usually hyphenated. Some examples: wide-angle, wide-eyed, wide-awake, wide-open. Exception: widespread.
An exception to Merriam-Webster:
co- Use a hyphen for nouns, adjectives and verbs that indicate occupation or status: co-author, co-chair, co-defendant, co-host, co-pilot, co-star, co-worker.
As part of a formal title before a name: co-President Alexa Manola, co-Executive Director Alfredo Hudson. Use no hyphen in other combinations: coeducation, coexist, cooperative (but co-op), copay.
Cooperate, coordinate and related words are exceptions to the rule that a hyphen is used if a prefix ends in a vowel and the word that follows begins with the same vowel. Co-op retains the hyphen to avoid confusion with a chicken's home.
No hyphen in coworking when the meaning is sharing workspace and amenities, such as Wi-Fi, a printer, fax machine and the like, when people don't actually work for the same company but instead are self-employed or remote workers.