Michael When forming a carpool, children and teenagers often start bickering about who gets to sit in the front passenger seat and who has to sit in the back. That's why in America, it's common for the person who first calls out "Shotgun!" to be allowed to sit in the front seat. There are different regional rules, but usually, it's only permitted to call "Shotgun!" when the car is within sight. In case of a dispute, the caller must prove this by pointing to the parked car.
The phrase Riding shotgun originates from the times of the Wild West, when a stagecoach would have an armed guard riding alongside the driver on the coach box during secured transports. In illustration 2, you can see a scene from the movie "Stagecoach" with John Wayne, showing the driver on the left and the guard with the rifle on the right. According to Wikipedia, one of the first mentions of the term "Riding Shotgun" can be found in the book "The Sunset Trail" by Christopher Keeney from 1905. Even today, "Riding Shotgun" refers to sitting in the front passenger seat, and "Calling Shotgun" refers to claiming that seat.