In the US, there is a longstanding tradition of determining who gets to sit in the seat next to the driver of a motor vehicle. In order to claim that seat, one must "call shotgun" according to a set of standard rules. The seat next to the driver is called shotgun in remembrance of those men that rode on stage coaches next to the driver with a shotgun to protect the stage from banditos (highwaymen). Or so it's said.
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, and a number of websites claim to have the official rules.
The purpose of the tradition, and the rules surrounding it, are to reduce conflict. In Americann culture, generally lacking in formal social rules, breaking shotgun rules is nonetheless considered a grave faux pas.
