The apostrophe has several functions, though it is most commonly used to form contractions and to indicate possession. Contractions combine words by omitting letters, as in don’t, can’t, and won’t. Possession is shown by adding an apostrophe and s to singular nouns, such as Andy’s car, Gareth’s cats, or the dog’s bone. This should not be confused with simple pluralisation. Adding an s to make a word plural does not require an apostrophe; examples include books, bananas, cats, and toys.

Guidance on possessive forms can vary between style authorities. One widely accepted principle is to add ’s to singular nouns, even when the word ends in s, resulting in forms such as Charles’s, Burns’s poems, or Dickens’s novels. Some references suggest omitting the additional s in specific cases where words end in certain sibilant sounds, such as Kansas’ Governor or Texas’ population. However, a consistent approach of adding apostrophe s to singular names ending in s is often regarded as the clearest and most straightforward solution. Pronominal possessives—hers, its, theirs, yours, and ours—do not take an apostrophe. Indefinite pronouns, however, do show possession with an apostrophe, as in one’s rights or somebody else’s umbrella.

Apostrophes are also used in expressions of time or quantity, such as a day’s journey, two weeks’ notice, or four yards’ worth. In plural possessives, the apostrophe follows the s (the girls’ dresses), whereas irregular plurals place the apostrophe before the s (children’s playground, women’s rights). Additional complexity arises in the pluralisation of abbreviations, letters, and figures, where apostrophes may be used for clarity—examples include M.D.’s, 747’s, or p’s and q’s. By contrast, shortened word forms and words discussed as words generally omit the apostrophe in the plural, as in co-ops, condos, ifs, ands or buts, and dos and don’ts.