The D-pillar of a car refers to the part of the car's structure that runs along the rear side windows, connecting the roof to the body of the car. This structural element provides support and stability to the car's roof, giving it its distinctive shape. While D-pillar is the most commonly used term, there are several other synonyms used by car enthusiasts and designers. Some of these include C-pillar, rear quarter panel, sail panel and cant rail. Each of these terms refers to the same structural element, highlighting a different aspect of its shape or function.