A seat (14), particularly but not exclusively for a wheelchair, comprises a bottom rest (13, 60), a back rest support (18) rotatable relative to the bottom rest about an axis of rotation (100); and a back rest (22) slidably mounted on the back rest support and moveable therealong to vary its position relative to the bottom rest. The back rest is kinematically linked to the bottom rest such that the back rest moves nearer to the axis of rotation as the angle between the back rest support and the bottom rest increases.