Or if the word ends with an s. Generally speaking, to make a noun a possessive – i.e., indicate ownership – you just add ‘s to the end. The dog’s leash got tangled. That’s Robert’s book. What is the Sarah’s address?
But if the noun is plural, it gets a little tricky. What if there’s more than one dog whose leash got tangled? You still add the apostrophe at the end of the word, but you do not add an s after it: The dogs’ leashes got tangled. That’s the boys’ book. What is the Millers’ address?
Most people want to add the s after the apostrophe. Not only is it grammatically wrong, it makes the word much more difficult to say. The dogs’s leashes…? The Millers’s address…? How do you even pronounce those?