Take a look at the side windows on your car, and you might notice a small triangular pane of glass beside the main window. It's easy to overlook, especially because it usually doesn't open or roll down. But that little piece of glass is not just there to make the car look a certain way; it can serve a few practical purposes.
The answer depends on where the window sits and whether it's fixed or movable. On modern cars, these small panes can help improve visibility and allow designers to shape the doors around the window mechanism. Older cars had a similar feature for a different reason: letting fresh air into the cabin long before air conditioning became standard.
