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POSSESSION

Most of the time, apostrophes are used to show possession.

However, in some cases they are used to show omissions, form contractions, or form the plural of acronyms and words being named.

Use 's  for the possessive of nouns not ending in s: 

• Singular: child’s shoes, woman’s shoe, dog’s bone. 
• Plural: children’s shoes, women’s shoes, cacti’s thorns. 

Use 's  for the possessive of singular nouns that end in s: 

• Charles’s books, Dickens’s novels, actress’s script. 
  Note: If a singular noun ending in s is followed by a word beginning with s, use only the apostrophe, not the ‘s. For example, Charles’ shirt.

Use ' without an s  to form the possessive of plural nouns that end in s:

• The Joneses’ car. The Dickenses’ home. The actresses’ costumes.

Use 's  for the possessive of indefinite pronouns:

• Anybody’s, somebody else’s, neither’s. 

Use 's  with only the last noun for joint possession in a pair or series: 

• The author and the illustrator’s contract. (The two jointly have one contract.)
• The author’s and the illustrator’s contracts. (The two have separate contracts.)

Use 's  to show omissions of form contractions:

• Omission of letters: O’ clock, jack-o’-lantern.
• Contractions: we’ll, don’t, weren’t.

Use 's  to form the plural of acronyms: 

• Three IRS’s, two BMW’s

Note: Plurals for numerals, letters, and years may be written with or without apostrophes as long as there is no confusion with other words (u’s, not us): three 7’s or three 7s, four c’s or four cs, the 1960’s or the 1960s. Just be consistent!