An apostrophe ( ' ) is used to indicate the omission of one or more letters in a word (rock 'n' roll), omission of a century in a year ('97), a contraction (they're) and to show possession.
When forming possessives of nouns
Plural nouns not ending in s, add 's: the women's group, the men's input.
Plural nouns ending in s, add only an apostrophe: the players' practice facility.
Nouns plural in form, singular in meaning, add only an apostrophe: mathematics' rules, Bronco athletics' code of conduct.
Singular nouns not ending in s, add 's: the University's needs.
Singular common nouns ending in s, add 's unless the next word begins with s: the hostess's invitation, the hostess' seat.
Singular proper nouns ending in s, add only an apostrophe: New Issues' first publication.
Compound words, add an apostrophe or 's to the word closest to the object possessed: the general counsel's request.
With compound nouns, placement of apostrophes for possession depends on whether the nouns are acting collectively or separately.
Jim's and Mary's weddings were both in Kalamazoo.
They are not married to each other (two separate weddings).
Jim and Mary's weddings were both in Kalamazoo.
Jim and Mary have been married twice to each other.
An apostrophe following the last name in a series indicates collective possession.