More Possessive Rules:

1. If the possessor is a person, we use ‘s:

Peter’s car

My mother’s cat

My friend’s house

1a. If the person is a plural with s (like "friends"), then just use the (') mark.

My friends' cars

His sons' rooms

But...you turn things around for rule #2:

2.  If the possessor is an object, we use “of”

The window of the office    or....

The window of this office 

Had enough? Return?