Here are the rules that will apply:

• Rule one: If possessive noun is singular, then you need to add apostrophe plus “s”.
• Rule two: If possessive noun is not ending with the letter “s”, then you always need to add apostrophe plus “s”.

The Possessive Apostrophe

• 1. Add apostrophe s to the end of a singular noun that does not end in s: the manager’s room
• 2. Add apostrophe s to the end of a singular noun, even if it ends in s (this practice may vary in some places): Doris’s scarf