Apostrophes are used in English to indicate (a) the possessive case of nouns and some pronouns, (b) the place where letters or figures have been left out of contractions, and (c) the plurals of some abbreviations.
Note that possessive nouns function as modifiers of the noun that follows them.
With singular nouns, the possessive is formed with an apostrophe followed by -s:
|A piranha||A piranha’s main interest|
With plural nouns, the possessive is formed by adding the apostrophe to the plural ending—to the -s that’s already there:
|the piranhas||the piranhas’ complaints|
|the witches||the witches’ contracts|
When the plural does not end in -s, the possessive is formed by adding an apostrophe plus s to the plural:
|children||the children’s hour|
|geese||the geese’s garden party|
Apostrophes are used to show where letters or figures have been left out:
|we are — we’re||they are — they’re|
|cannot — can’t||do not — don’t|
|1995 — ’95||it is — it’s|
Plurals of Coined Words and Abbreviations
When words or letters are being talked about, their plurals are formed by adding an apostrophe plus s:
The that‘s in this sentence have been ellipted.
He crosses his i‘s and dots his t’s.
The plurals of abbreviations with internal periods are formed with an apostrophe; those without internal periods without an apostrophe:
an M.P.—the M.P.’s lounge
one Ph.D. is plenty—two Ph.D.’s are sure to pontificate.
All the RCMPs in Ottawa couldn’t make him change his socks.