An apostrophe. The first rule for using an apostrophe is to show possession, belonging or attachment. For example: As a child, if I had done anything wrong at school I would have been sent to the Principal’s office. That is, the office belonging to the Principal. Be aware that when the apostrophe follows an ‘s’, at the end of the word. For example: The boys’ room (the room belonging to John and Jack) Jess’ car (the car belonging to Jess).