Abstract: An assembly jig includes a first frame adjustably fastened to a vehicle body in the vicinity of a door opening, a second frame with supporting the door of the vehicle, and link arms between the first and second frames that are hinged to the first and second frames so as to form a deformable quadrilateral in association therewith. Hinges between the arms and the frames include at least one ball joint arranged between a carrier element secured to one of the frames or arms and a carried element secured to one of the arms or frames, carrying being relative to the effect of gravity, wherein at least one of the carrier and carried ball joint elements is secured to the corresponding arm or frame in such a manner that the position in three-dimensional space of the center of the ball joint is adjustable in three spatial directions.