An improved signature handling apparatus includes a movable gripper jaw which operates in conjunction with similar gripper mechanisms to clamp and spread the signature so that the laps fall on opposite sides of the saddle in a saddle gatherer. In order to assure that the pages on the opposite sides of the fold fall on the appropriate side of the saddle gatherer, a sensing means is utilized to detect whether all of the pages on one side of the signature are properly clamped by the gripper jaw. Since the position of the gripper jaw is indicative of whether the correct thickness has been clamped, the sensing means emits a positive response when the gripper jaw is in the correct position. The sensing means includes a part operatively associated with the gripper jaw which is adjusted by connecting means to be in a predetermined position when the gripper jaw properly clamps the signature leg. Depending on the embodiment, the operatively associated part has either a reflecting surface or an aperture, as means for sensing its position. The sensing means further includes a light source and light sensor whereby light is received into the light sensor after either being reflected from the reflecting surface or passing through the aperture depending on the embodiment, only when the operatively associated part is in the predetermined position. Upon receiving light from the light source, the light sensor emits an electrical impulse indicating that the signature lap has been properly clamped.