Body in White or BIW refers to the stage in automotive design or automobile manufacturing in which the car body sheet metal (including doors, hoods, and deck lids) has been assembled or designed but before the components (chassis, motor) and trim (windshields, seats, upholstery, electronics, etc.) have been added.
The name is derived from manufacturing practice before steel monocoque bodies, sometimes trademarked unibody. When most cars were made by firms as just a frame, with an engine, suspension, and fenders attached, the manufacturers built or purchased wooden bodies (with thin, non-structural metal sheets on the outside) to bolt onto the frame. The bodies really were painted white as a preliminary to being painted the customer's chosen color. During the Great Depression, firms had a large inventory of these bodies and took a long time to sell them off. Now that car bodies are made of steel, the phrase remains and has a folk etymology that it comes from the appearance of the car body after it is dipped into a white bath of primer (undercoat paint)—when actually the color is usually light gray.
In car design, the Body in White phase refers to the phase in which the final contours of the car body are worked out, in preparation for ordering of the expensive production stamping die. Extensive computer simulations of crash worthiness, manufacturability, and automotive aerodynamics are required before a clay model from the design studio can be converted into a Body in White ready for production