Here are the general rules to follow when forming the possessive of plural and compound nouns.

To form the possessive of plural nouns ending in -s or -es, add an apostrophe (').

Examples:

• pupils' costumes
• witches' brooms

To form the possessive of plural nouns not ending in -s, add an apostrophe and an -s or -'s.

Example:

• children's books

To form the possessive of compound nouns, add an apostrophe and an -s or -'s to the last word.

Example:

• maid-of-honor's gown