Abstract: A device for releasably holding a movable body in a given position with respect to another body includes a pair of opposed mutually spaced end walls and a pair of opposed mutually spaced fixed walls extending between the fixed to the end walls to form therewith a frame which surrounds a given space. A control plate is situated in the latter space for movement between the end walls in a clamping direction or in a non-clamping direction, and on opposite sides of the control plate are a pair of parallel clamping plates which are parallel to the control plate while being spaced therefrom.