among other things , an open - ended wrench comprises a wrench head defining an opening to receive a work piece . the wrench head comprises a first jaw including at least a first surface to engage with a surface of the work piece , a second jaw including a first curved surface facing the opening , and a plate mounted on the second jaw and movable relative the second jaw . the plate comprises at least a second surface to engage with another surface of the work piece . the plate also comprises a second curved surface engaging the first curved surface and being slidable against the first curved surface .