The purpose of the tradition, and the rules surrounding it, are to reduce conflict. In American culture, which is generally notably lacking in the formal social graces, breaking shotgun rules is nonetheless considered a grave faux pas. Most people consider the legitimate shotgun call to be sacrosanct.
To call shotgun is to, at minimum, yell out "shotgun" while approaching the vehicle. The specific rules are the subject of much heated but good natured debate. A number of websites claim to have the official rules, but clearly only a free and open consensus site such as Wikipedia has the moral and intellectual right to make that claim.
Therefore, by the powers vested in Wikipedia by the present King of France, the following rules are hereby decreed: