The small pane beside the windshield
The triangular pane is usually called a front quarter window. It sits between the windshield, the door’s main window and the car’s front pillar—the structural post that supports the roof. Without it, that junction could leave a wedge-shaped area of glassless space or require a different, bulkier door-and-windshield layout.
It also helps the driver see around the front pillar. Pillars have to be strong, but they can hide nearby pedestrians, cyclists or traffic. A small pane in the space beside the pillar gives the driver another angle through the car, especially when checking the area around the side mirror. It does not eliminate the pillar’s blind spot; shifting position and looking around the pillar still matter.
When the window could open
On many older cars, the pane was mounted in a frame that pivoted outward. Angled into the oncoming air, it could direct a stream into the cabin without rolling down the much larger door window. That made it useful before air conditioning was common, and even afterward for drivers who wanted fresh air aimed at them.
The design had a tradeoff: a hinged pane and its frame added hardware, and an open vent could bring in noise and rain. As cars gained more effective heating and air conditioning, the need for a dedicated air scoop faded. Designers also favored smoother, more integrated window shapes, so the little pane often became fixed—or disappeared in some models.
A fixed triangle still has a practical job: it lets glass occupy the narrow space next to the front pillar, preserving a useful sightline without making the main door window unusually shaped.
