The core (1) of a seat back, which will be upholstered to form the complete back, is a moulded plastics sheet bowed forwardly and with attachment points (12) near top and bottom for securing to a rigid support. The sheet is constructed to be stiffer over its upper portion then over the lower and, when secured, to be able to ripple or alter its curvature between the attachment points. It may have wings (2) inclining forwardly at each side with closed end slots (3) between the wings (2) and the central zone (8) of the core. The wings can be divided into generally horizontal sections carried by integrally moulded transverse ribs (4) spanning the slots (3), the division between sections being by slots (13) or narrow membranes (6). The top of the core may also be divided into side-by-sections, the division being by slots (14) or narrow membranes (11).