+556.+ The apostrophe is used to indicate the omission of letters or
syllables, as: _He doesn't_, instead of _does not_; _We're_, instead of
_we are_; _I'm_, instead of _I am_; _ it's_, instead of _it is_;
_ne'er_, instead of _never_; _they'll_, instead of _they will_, etc.
+557.+ The apostrophe is also used to denote possession. In the single
form of the nouns it precedes the _s_. In the plural form of nouns
ending in _s_ it follows the _s_. For example:
Boy's, man's, girl's, king's, friend's, etc.
Boys', men's, girls', kings', friends', etc.
Note that the apostrophe is not used with the possessive pronouns
_ours_, _yours_, _its_, _theirs_, _hers_.
+558.+ The apostrophe is used to indicate the plural of letters, figures
or signs. For example:
Dot your _i's_ and cross your _t's_.
He seems unable to learn the table of 8's and 9's.
Do not make your _n's_ and _u's_ so much alike.
+559.+ The apostrophe is used to mark the omission of the century in
dates, as: '87 instead of 1887, '15 instead of 1915.