C-pillar
The vertical support dividing a car's rear window from its rear windscreen, or, if the car has an additional window and D-pillar, the support dividing the central and rear windows.
C-pillar: the structural post behind the rear doors
The C-pillar is the vertical structural member of a car's bodywork that sits behind the rear passenger door and supports the roof. On sedans and most hatchbacks, it forms the rear edge of the passenger cabin and separates the rear side window from the rear windscreen. On longer vehicles like extended SUVs or certain wagon designs, a fourth pillar (D-pillar) may exist behind it, in which case the C-pillar divides the rear side window from a centre window instead.
C-pillars carry significant structural load. They anchor roof rails, support the rear door hinges through the B-pillar bracing behind them, and form part of the safety cage that protects occupants in side-impact collisions. Modern C-pillars are typically welded box sections or tubular members, often formed from high-strength steel or aluminium to keep weight down while maintaining torsional rigidity. The cross-sectional area is optimised to resist bending and crushing without excessive mass.
Common problems and design trade-offs
Rear visibility is a persistent challenge. A thick C-pillar reduces the window opening available behind the rear door, creating a blind spot that can hide pedestrians, cyclists, or other vehicles. Designers balance safety cage strength against sightlines, sometimes using thinner glass or wider rear windows to compensate. Some vehicles employ convex glass or camera-based blind-spot warning systems to mitigate this trade-off.
Crash worthiness also depends on C-pillar orientation and cross-sectional stiffness. In side-impact testing, a weak or poorly aligned C-pillar can buckle or collapse, allowing the intruding vehicle to penetrate deeper into the cabin. Regulatory crash tests measure pillar deformation and occupant injury metrics; high-strength steel grades and optimised geometry are standard in modern designs to meet these standards.
The name follows a simple alphabetical scheme: A-pillar at the front (behind the windscreen), B-pillar in the middle (between front and rear doors), C-pillar behind that, and D-pillar at the very rear if one exists. This terminology is universal across the automotive industry and appears in engineering drawings, service manuals, and crash test reports worldwide.