Use apostrophes for three purposes: to show possession, to indicate an omission, and to add an ending to a number, letter, or abbreviation.

1. To make a singular noun possessive, add '-s.

   
   

   The surgeon's excellent reputation calmed Larissa's fears.




2. To make a plural noun ending in -s possessive, add an apostrophe.

   
   

   The researchers' instruments were not properly sanitized.




3. To make a plural noun not ending in -s possessive, add -'s.

   
   

   The temple offers a divorced women's support group.




4. Use an apostrophe to indicate an omission in a contraction.

   
   

   I can't meet you at the gym; I've got too much homework tonight.




5. Use an apostrophe to form the plural of an abbreviation and of a letter, word, or number mentioned as a word.

   
   

   My aunt is a member of the ACLU's advisory board.

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