A homophone is a word that sounds the same as another word but has a different meaning, and usually a different spelling. There, their and they're are homophones. So are to, too and two, and its and it's. The name comes from Greek: homos, "same", and phōnē, "sound".
Homophones are not a problem when you speak, because the listener hears one sound and picks the meaning from the sentence. They become a problem the moment you write, because the page has to commit to one spelling.
Homophones, homographs and homonyms
Three related words get mixed up almost as often as the homophones themselves:
- Homophones sound alike: right and write, piece and peace.
- Homographs are spelled alike but differ in meaning, and sometimes in sound: lead the metal and lead the verb, a bass guitar and a bass in the river.
- Homonyms is the loose umbrella term, used for words that share a sound, a spelling, or both, such as bank (of a river) and bank (for money).
In everyday writing advice, "homophone" means the troublesome kind: two spellings, one sound.
The homophones people mix up most
- there (a place: put it there), their (belonging to them: their car), they're (they are: they're late)
- its (belonging to it: the dog wagged its tail), it's (it is, or it has: it's been a long day)
- your (belonging to you), you're (you are)
- whose (belonging to whom), who's (who is, or who has)
- to (direction, or part of a verb: to go), too (also, or excessively), two (the number)
- principal (main, or the head of a school), principle (a rule or belief)
- complement (something that completes), compliment (praise)
- stationary (not moving), stationery (paper and envelopes)
Some pairs are near-homophones rather than exact ones, but they cause the same trouble: affect and effect, accept and except, then and than. Accent matters too: a pair that sounds identical in one part of the English-speaking world can be clearly different in another.
A test that settles the apostrophe pairs
Half of the list above is a contraction against a possessive. For those, expand the contraction and read the sentence again:
- It's raining becomes it is raining, which works, so the apostrophe stays.
- The company changed it's name becomes changed it is name, which does not, so it should be its.
- The same works for who's (who is), you're (you are) and they're (they are).
Possessive pronouns never take an apostrophe: its, whose, yours, theirs. More on that in How to Use Apostrophes.
Why a spelling check misses them
A spelling check compares each word against a dictionary. Their is in the dictionary. So is there. Looked at one word at a time, "Their going home early" contains no spelling mistake at all. Catching it means reading the word in its sentence: their is followed by a verb, where only they're makes sense.
How the spell checker helps
The Spell Checker checks grammar as well as spelling, and its grammar rules look at the words around a homophone. Many of the common mix-ups above are flagged, with the other spelling offered as the fix, so one click puts the right word in.
No automatic check knows what you meant in every sentence, though. For writing where the meaning decides which word is right, the AI Spell Checker reads the text in context, and a last read of your own is still worth the minute it takes. See What Is Proofreading? for a routine that catches what tools miss.