Apostrophe

DO use an apostrophe to denote the omission of one or more letter in a word.

I don't think this ad is so outstandin'.

DO use an apostrophe to form the plurals of letters, numbers, or words.

There were several 6's in the list.

DO use an apostrophe, followed by an s, to indicate the possessive form of most singular nouns or pronouns.

This is David's work.

EITHER use an apostrophe after the final s or add an apostrophe and another s to indicate the possessive form of singular nouns ending in an s, sh, or z sound.

It is James' house ... or ... It is James's house.

ONLY use an apostrophe after plural nouns ending in an s, sh, or z sound, to indicate the possessive form.

It is the Thomases' car.

DO use an apostrophe, after the s, to indicate the possessive form of most plural nouns.

The students' ads were too simple.

DO use an apostrophe and an s immediately before the word being modified, to indicate the possessive form of compound nouns.

The Attorney General's action was expected.

DO NOT use an apostrophe to indicate possession for personal or relative pronouns.

Its message was clear ... NOT ... It's message was clear.