The apostrophe has three primary uses in the English language: to form possessive nouns, to represent one or two omitted letters in contractions, and to indicate plurals or lowercase letters.

Possessive Singular Nouns

To form a possessive of a singular noun, including nouns ending in s, x or z, add an apostrophe followed by an "s" at the end of the noun. For example: the dog's leash, my friend's father, the boat's deck, or 2009's economic downfall.

Form Possessive Plural Nouns

To make a plural noun possessive, add only an apostrophe to the end of the plural noun, such as the politicians' votes not politicians's votes or Rodriguezes' house not Rodriguezes's house.

Pronominal Possessives, Compound Words & Multiple Nouns

Pronominal nouns, such as hers, its, theirs, yours, and ours, do not need an apostrophe to make them possessive. Indefinite pronouns need the apostrophe to make them possessive, for example, one's or else's.

For compound words such as sister-in-law, add the apostrophe and "s" to the last word--sister-in-law's--to make it possessive. Another example is attorney general, which becomes attorney general's not attorney's general.

If two or more entities possess something independently of the other, make them both possessive, such as my sister's and my mother's feelings. If two or more entities possess something together, add the possessive to the last noun, such as student and teacher's project.

Italics and Quotation Marks

If the possessive noun is italicized, such as a magazine's name, the apostrophe and "s" should not be italicized. If the possessive noun is in quotation marks, the apostrophe "s" does not go inside the quotation mark. In the latter scenario, reword the sentence.

Special Situations

Singular nouns ending in "s" use only an apostrophe, not the apostrophe and "s" to make them possessive. Other examples of using only the apostrophe include grammatically singular nouns that name a group or collective entity (such as politics' or United States'), or names that end with a silent "s" (such as Descartes' or Albert Camus'), or names that end in an unaccented ending pronounced "eez" (such as Aristophanes' or Charles Yerkes') and nouns that end in an "s" or "s" sound.

In "For ... sake" sentence structures (for conscience' sake or for appearance' sake) use just an apostrophe at the end of the noun to make it possessive.

Contractions

When forming contractions, such as it's, can't or won't, the apostrophe indicates an omission one or two letters. For example, "I am" becomes "I'm," "it is" becomes "it's,""can not" becomes "can't," and "will not" becomes "won't." For a numerical value indicating a year such as 1960 can be shortened to '60 with the apostrophe representing the omitted numbers.

Plurals of Lowercase Letters

The apostrophe is also used to indicate plurals of lowercase letters, such as t's when referring to multiple or all examples of the letter t, such as "all t's" or "the three t's."