The term “Shotgun” refers to the front passenger seat of an automobile. “Calling Shotgun” is the act of claiming the position of Shotgun for one’s self. As this position is the most coveted of all positions when riding in a car, the following list of rules has been created to ensure that Shotgun can be acquired in a fair and equitable manner by any passenger of an automobile.
I never knew there were official rules, but surprisingly, they hardly differ at all from what I’ve always accepted.
Check out The Official Shotgun Rules. You can even buy a pocket reference for your glove-compartment!