Are you sure you mean the 'A-Frame', or do you mean the 'A-Pillar'?
The frame of the car supports the door pillars and the roof. It supports the roof on two or three pillars depending on whether the car is a two door or a four door.
Immediately after the hood (bonnet), the A-Pillar supports the hinges for the front doors and the windshield (windscreen). The B-Pillar supports the pin that secures the front door locks and the hinges for the rear doors (if fitted). The C-Pillar supports the pin that secures the the rear door locks (if fitted) and the rear window. Each pillar is identified by the side on which it is found (driver side / passenger side) and the the pillar location: ex. Driver's A-Pillar or Passenger B-Pillar.