A seat having a hinge mechanism and a disabling system enabled to selectively allow and prevent a backrest from pivoting in relation to a seating portion. The hinge mechanism may have a case, a locking system, and a control member. The locking system may be locked and unlocked to prevent and enable pivoting of the backrest. The control member may be moveable between a rest position, locking the locking system, and an actuating position, unlocking the locking system. The disabling system may have a disabling member and a retaining member. The disabling member may have an active position, placing the control member in the actuating position. The retaining member may maintain the disabling member in the active position. The case may contain, in a radial dimension, at least half of both the disabling member and the retaining member.