Body-on-frame construction is the foundational building method used in virtually every F-100 ever produced. The truck consists of two primary structures: a steel ladder frame that carries the engine, transmission, suspension, and drivetrain, and a separate body (cab and bed) that mounts on top of the frame using rubber body mount bushings. The only exception was the 1961-1963 Unibody option, which integrated the bed into the body structure while still using a frame under the cab.
This construction method matters to F-100 enthusiasts for several practical reasons. It makes body swaps and engine swaps significantly easier since the body can be lifted off the frame to access components. A rusted cab can be replaced without disturbing the running gear, and a good body can be moved to a better frame. Many restoration projects begin by separating the body from the frame to address rust, repaint, and upgrade components independently.
Body-on-frame design also makes the Crown Vic swap and other suspension modifications feasible, since the frame can be cut and modified without affecting the body. The rubber body mounts between frame and body absorb vibration and should be inspected and replaced as part of any thorough restoration.