Abstract: A recline unit (6) for controlling the recline angle of the back rest (2) of a seat (1) is interposed as a strut between the back rest (2) and the seat base (3). The recline unit (6) has a recirculating ball screw (7) and nut (8) of which the former is mounted to the back rest (2) so as to be restrained against rotation and of which the latter is mounted to the seat base (3) so as to be restrained from moving axially but to be free to rotate. Manual pressure applied to the back rest (2) to incline it will move the ball screw (7) axially and cause the nut (8) to rotate. A locking mechanism (25) is provided to lock the nut (8) against rotation when the back rest (2) is in the desired reclined attitude and so prevent further axial movement of the ball screw (7). The occupant of the seat (1) can unlock the locking mechanism (25 ) by operating a button (40) which acts through a pull cable (37) on the locking mechanism (25).