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Body in white is the stage in automobile manufacturing in which a car body's frame has been joined together, that is before painting and before the motor, chassis sub-assemblies, or trim have been integrated into the structure. Assembly involves different techniques such as welding , riveting, clinching, bonding and laser brazing.
The term derives from manufacturing practices before steel unibody monocoques — when automobile bodies were made by outside firms on a separate chassis with an engine, suspension, and bumpers attached. The manufacturers built or purchased wooden bodies to bolt onto the frame. The bodies were painted white prior to the final color.
A folk etymology for "body in white" is the appearance of a car body after it is dipped into a white bath of primer — despite the primer's actual gray color. BIW could also refer to when car bodywork would be made of timber - all timber products, furniture, etc., are considered to be "in the white" when at the stage of raw timber before finishing or varnishing.
In car design, the "body in white" phase is where the final contours of the car body are worked out, in preparation for the 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.