Abstract: A device for lying on or sitting on comprising a mattress (3) with a mattress sheath (4), a lath grid (1, 2) and at least two springy beams (5) extending in longitudinal direction of the device, wherein the lath grid (1, 2) comprises a lath grid cloth (1) with means for fixing laths (2) to one side of the lath grid cloth (1) in a transverse direction of the device and parallel to each other, and wherein the mattress (3) and the lath grid (1, 2) are connected by two detachable connection means extending in a longitudinal direction of the device, to form a mattress/grid-unit being substantially wholly covered with cloth, wherein the two connection means comprises each a pair of cooperating parts (14, 44) of which parts (14, 44) a first part (44) is fixed to the mattress sheath (4) and a second part (14) is fixed to the lath grid cloth (1).