Place an apostrophe followed by the letter "s" after a singular noun to form the singular possessive. For example, a bag belonging to Judy would be Judy’s bag. If the noun ends with an "s," you also have the option of just placing the apostrophe after the "s"; for example, Charles’ dog. For nouns ending in z or x you can add an "es" to the end followed by an apostrophe, the extra syllable makes the word easier to pronounce.
Add only an apostrophe to a plural noun that ends in the letter "s" to form the plural possessive (nurses’ uniforms or farmers’ barns). This shows the reader that more than one person owns the item possessed.
Add an apostrophe and then the letter "s" to a plural noun that doesn’t end with the letter "s" (children’s toys, women’s skirts).
Add the letter "s" to the pronoun "it" to form the singular possessive. For example, "The company found its niche market." This is the exception to the general rule that a possessive takes an apostrophe. There is no plural possessive for "its."
Add the apostrophe and then the "s" to the end of compound words. In a compound construction indicating joint ownership the possessive form applies only to the second noun. For example, Josh and Amber’s anniversary party. This is the case because Josh and Amber share the anniversary party.
Use a double possessive when talking about individual ownership for two people. For example Megan’s and Audrey’s hairstyles. This is the case because Megan and Audrey each have their own hairstyle.
Use the word "its" when describing ownership of things. For example, "The house was old, its windows were shabby." The word "it's" is not a possessive but a contraction. "It’s a bad idea to skate on thin ice" is a contraction of "It is a bad idea to skate on thin ice."
Use personal pronouns like his, hers, theirs, yours and ours on their own, never use an apostrophe with these pronouns. With these pronouns ownership is already implied.