An assembly (1) between a side member (2) and a rear panel (3), for instance in a piece of furniture, a wardrobe or a kitchen element. The assembly consists of a flange part (3a) of the rear panel (3) inserted sidewards into a locking slot (6a) in the side member (2), the insertion of the flange part (3a) being effected via a longitudinal slot (8) at the access to the locking slot (6). The rear panel (3) is secured to the side member (2) by at least one part (10) of the side member (2) extending into at least one withdrawal slot (20) in the rear panel (3), or by at least a part (21) of the rear panel extending into at least one withdrawal slot (23) in the side member (2). Hereby an assembly is obtained which is not only light of weight, but which is also solid and robust and easy to set up.