1. To show possession in nouns, indefinite pronouns and adjectives
Examples:
the boy’s dog (singular noun), the boys’ dogs (plural noun), the children’s
coats
(irregular
plural noun), no one’s fault (singular indefinite pronoun), a day’s
pay (singular
sense adjective),
three months’ maternity leave (plural sense adjective)
2. To show the omission of letters in contractions
Examples: doesn’t (does
not), haven’t (have not), hasn’t (has not), don’t (do not),
I’d
(I would), she’s (she is), it’s (it is)
3. To form the plurals of numbers and letters
Examples: 10's, 100's, the 1970's, mind your p’s and q’s, write your a’s and b’s
4. To show the omission of numbers, especially in writing years
Examples: the class of ‘73, the team of ‘72, sweet ‘45