So interesting how different things are in different regions. I've never heard of a back doorbell before. At one grandmother's house, we did always use the "back door" because of how her house was situated on a bit of a hill. The driveway led to the back door (actually on the side). People who weren't family/friends would park on the street in front and walk up the hill to the front door, which had a doorbell. We always just knocked on the back door or, more commonly, just walked in, since she always left it open if anybody was coming by. Every other house I can remember going to, everybody used the front door. Seems weird to go poking around someone's backyard when they don't even know you're there.