On a car, the B pillar begins at the passenger-side and driver-side windows. This pillar is generally present on four-door and five-door vehicles, but some vehicles with two and three doors have them too.
Someone cannot see the B pillars until they open the car doors. The frames on the front-door windows of a car generally hide the B pillars when the doors are not open. These pillars are vertical and provide a place for the doors to hinge onto when someone closes the doors on a vehicle. This pillar also serves as a place the attach the mountings of the seat belts in the front seat of a car.