Pillars are the vertical or near vertical supports of a car's window area or greenhouse—designated respectively as the A, B, C or (in larger cars) D-pillar, moving from front to rear, in profile view.

The consistent alphabetical designation of a car's pillars provides a common reference for design discussion and critical communication. As an example, rescue teams employ pillar nomenclature to facilitate communication when cutting wrecked vehicles, as when using the jaws of life.^[1]

The B pillars are sometimes referred to as "posts" (two-door or four-door post sedan).^[2][3][4]

In the case of the B (or center) pillar on four-door sedans, the pillar is typically a closed steel structure welded at the bottom to the car's rocker panel and floorpan, as well as on the top to the roof rail or panel.^[5] This pillar provides structural support for the vehicle's roof panel and is designed for latching the front door and mounting the hinges for the rear doors.^[5]

As the most costly body components to develop or re-tool, a vehicle's roof and door design are a major factor in meeting safety and crash standards.^[6] Some designs employ slimmer, chamfered windscreen pillars, A pillars, to help improve driver vision (thus reducing blind spots) through the use of stronger alloy steel in these components.^[6] As "perhaps the most complex of all the structures on the vehicle", the center or B-pillar may be a multi-layered assembly of various lengths and strengths.^[7]

Closed vehicles without a B-pillar are widely called hardtops and have been available in two or four-door body styles, in sedans, coupes and wagons.^[8] Designs without a center or "B" pillar for roof support behind the front doors offer increased occupant visibility, while in turn requiring underbody strengthening to maintain structural rigidity.^[9] In the early 1970s, General Motors broadened their definition of "hardtop" to include models with a B-pillar although: "up to then, everybody thought a hardtop was a car without a center pillar."^[10]

Pillars are implied, whether they exist or not; where a design's greenhouse features a break between windows or doors without vertical support at that position, the non-existent pillar is "skipped" when naming the other pillars. Thus a two-door hardtop or a three box designed coupé could have its rearmost pillar called the C-pillar even in the absence of a B-pillar. Conversely additional doors, such as on limousines, will create additional B-pillars; the B-pillars are then numbered, B1, B2, and so forth.

In addition to the pillar nomenclature derived from viewing an automobile in profile, some older cars have a two-part windshield or a split rear window, with the two halves separated by a pillar. Posts for quarter windows (a smaller window typically between the front window and the windshield) are not considered a named pillar.