A prefix is a group of letters that is added to the beginning of a word (known as the base word or stem word) to create another word.
Most prefixes are set solid with the base word:
| aero- (aeroplane) | electro- (electromagnet) | meta- (metadata) | pre- (preoperative) |
| after- (afterthought) | en- (encode) | micro- (microorganism) | pro- (pronoun) |
| ante- (antenatal) | exo- (exoskeleton) | mid- (midterm) | pseudo- (pseudofluid) |
| anti- (antibody) | extra- (extramarital) | milli- (millimetre) | re- (readmit) |
| astro- (astronaut) | geo- (geographic) | mini- (miniskirt) | semi- (semidetached) |
| auto- (autosuggestion) | hemi- (hemisphere) | multi- (multitalented) | stereo- (stereophonic) |
| bi- (bipolar) | hyper- (hyperactive) | nano- (nanoparticle) | sub- (subsystem) |
| bio- (biodata) | hypo- (hypoglycaemic) | non- (nongovernment) | super- (supermarket) |
| centi- (centipede) | in- (inflexible) | para- (paramilitary) | supra- (supraorbital) |
| chemo- (chemotherapy) | infra- (infrared) | photo- (photograph) | trans- (transgender) |
| co- (cochair) | inter- (interdisciplinary) | physio- (physiotherapy) | tri- (trilateral) |
| counter- (counterterrorism) | intra- (intravenous) | phyto- (phytoactive) | ultra- (ultrasound) |
| di- (diode) | iso- (isotope) | poly- (polyunsaturated) | un- (unflagging) |
| dis- (disinherit) | macro- (macroeconomics) | post- (postnatal) | under- (underestimate) |
When the base term consists of more than 1 word, use an en dash after the prefix instead of a hyphen (see En dash):
the post–French Revolution period pro–women’s rights
However, if the base term itself has a hyphen, use a hyphen after the prefix:
Exceptions for hyphens with prefixes
Use a hyphen after the prefix when:
- the word formed would otherwise be confused with another word
- the combination of the prefix and the base word would create an unacceptable double or triple vowel in the middle of the word
ultra-aerated not ultraaerated
anti-aircraft not antiaircraft
- the boundary between 2 parts of the word would otherwise be obscured
sea-legs not sealegs
non-event not nonevent
- the base word begins with a capital letter or is a number
- the prefix is e- (short for electronic), except for more established forms
- the prefix is ex- (meaning former)
- the prefix or first part of the word is cross, except for more established forms
- the prefix or first part of the word is self-
Did you know? Cooperate and coordinate started out with hyphens (co-operate, co-ordinate), to indicate that the 2 o’s did not form a double o.
These are now so familiar that confusion is unlikely, so they are set solid.