Some cars leave a paper trail; the GM D platform left a name and a place in GM (General Motors)'s list. What follows is what can be said with a source behind it.
The story
The GM D platform (informally, D-body), was a General Motors automobile platform designation, used in two series (1936–1984 and 1965–1996) for large body-on-frame rear-wheel drive automobiles. For the majority of its existence the D-Body represented the largest Cadillac, either the Fleetwood Series 75 or the Fleetwood Limousine. In 1985 GM downsized and redesignated its traditional large car rear wheel drive GM C platform as the new much smaller and lighter front wheel drive GM C platform. It also shifted its traditional, D platform Fleetwood limousine to the new C platform, ending production of the historic D platform.
In turn, it redesignated the former RWD C platform the new D platform, and maintained production of a few RWD holdovers on it. The original D platform was closely related to GM's other two rear-wheel drive large car platforms: the mainstream GM B and the upscale C platforms. But save for the limited production top-of-the-line 1936–1942 Buick Limited, the original GM D-platform was exclusive to Cadillac, GM's most luxurious brand. As GM's largest, the "new" D platform designated in 1985 was reserved to Cadillac through the end of its production in 1996.
Its place in the line
Its family tree is legible: assembly was at Detroit Assembly, Detroit, Michigan, U.S.. Nearby in the GM (General Motors) catalogue sit the 1955 Chevrolet and the 1957 Chevrolet.