Americans refer to the seat next to the driver in a car as the shotgun seat - this derives from the old stagecoach days when an armed guard with a shotgun sat next to the driver (or at least they did in old Western movies). This is the most coveted passenger seat in a car and to "call shotgun" is to claim the seat on the next stage of a journey. There are several amusing websites, laying down rules for calling shotgun - see, for example, http://www.laughnet.net/archive/misc/shotgun.htm