The rule governing the procedure for backseat vehicular seating arrangements. If three passengers are forced to sit in the backseat of vehicle containing three spots, the Zapple Rule applies. The first two passengers to yell 'zapple' are given a door-side seat. The remaining passenger is given the middle seat.
Jimmy: "Hey Joe, Frank, and Sam. My cousin Tim is riding in the front passenger seat, so you will all have to ride in the backseat of my 1996 Toyota Camry."
Sam: "Oh shucks, the Zapple Rule. I guess I will have to ride in the middle, but luckily, this Camry's backseat is spacious!"