Revelle Humanities Grammar Handbook
6 Sentence fragments
7 Run-on sentences
8 Verb agreement
13 Passive voice
15 ESL
17 Misplaced modifiers
18 Pronoun reference
20a Comparisons
21 Parallel construction
23 Commas
23d Sexist language
25/6 Semicolons & colons
29 Ellipsis
31 Brackets
35 Apostrophes
36 Abbreviations
37 Underlining
39 Numbers
41b Slang
41e Jargon
41f Pretentious language
42c Idioms
42e Clichés
43a Fluff

35 Apostrophes
Apostrophes are mainly used in contractions and to show possession.

Possession by a singular noun: Penelope's suitors showed no sense of the rules of hospitality.

Possession by a plural noun: The suitors' lack of hospitality caused great annoyance.

Contractions: Generally, avoid contractions in formal papers.

Note: Its is the possessive form of it. It's is a contraction for it is. Whose is the possessive form of who. Who's is a contraction for who is.

See page 209 of your Brief English Handbook for other examples.