The fourth frame parts (34) protrude upwards from the back ends (32) of the first frame parts (14) and are joined to them to form one piece. The transverse distance (36) between the two back fourth frame parts is greater than the transverse size of the two front second frame parts (22) or than the transverse size (30) of the seat surface (28). The back ends (24) of the two third frame parts (22) are joined together in one piece. The two first frame parts run in a straight line between their front (16) and back (32) end and form a surface for standing on which widens out from front to back. The top end sections (38) of the two fourth frame backs curve upwards and backwards for fixing the backrest (40).
