Since I am the oldest, growing up I got the privilege of sitting in the front seat first, because the other two had either booster seats or were too young. This was great until Niamh and Aoife started growing...and realized that the front seat wasn't actually Aisling's seat. I had always known about calling shotgun, but one day after Niamh and I had many fights over the front seat (ending up with us both sitting in the back), she ran out of the house yelling "SHOTGUN SHOTGUN."
It was war.
The rules of shotgun (according to us) are this:
1. If mom and dad are both in the car, nobody gets shotgun.
2. All of us have to be outside the house and the car has to be in sight in order to call it.
3. You can't have shotgun to AND from the destination (I usually get away with this one)
4. If your hand touches the door, you can ride shotgun.
5. Mom has the overall say of who rides shotgun.
Aoife is now grown up enough to know that she too can call shotgun, but with 3 of us, we fixed the rules to our standards, and most days we get punished so we all have to sit in the back.