A piece of furniture (1) has a first piece (4) attached to second and third pieces (2) being perpendicular to the first piece (4) and where two opposing edges (5) of the first piece (4) are attached to one of the second and third pieces (2) each. The edges (5) of the first piece (4) are received in a groove (10) of each of the second and third pieces (2), that the first piece (1) is perpendicular to the second and third pieces (2) and that the second and third pieces (2) are placed parallel to each other. The mounting of a back of a cupboard is made in that two opposing vertical edges of the back is received in a vertical groove each at the edge of two side pieces, which two side pieces are arranged at the two opposing vertical edges of the back.