A kit car is an automobile available as a set of parts that a manufacturer sells and the buyer then assembles into a functioning car. Usually, many of the major mechanical systems such as the engine and transmission are sourced from donor vehicles or purchased new from other vendors. Kits vary in completeness, consisting of as little as a book of plans, or as much as a complete set with all components to assemble into a fully operational vehicle such as those from Caterham.