Here are a couple of examples:
The twins’ style is so different, even though they are identical.
When you’re forming a possessive, and the item (or person) doing the possessing ends in “s,” you add the apostrophe to the end of the word.
Conversely, if the word doesn’t end in “s,” add one and put the apostrophe before it.
Here’s how to remember it: No “s,” add one. With “s,” needs none.