An open wrench includes a body having a mounting hole which is partially defined by a first side face which has a curved reception recess and a cavity which communicates with the reception recess. A protrusion is formed at the intersectional area between the cavity and the reception recess. A first contact face is located in opposite to the first side face of the mounting hole so as to contact one side of the object. A second distance is defined between the reception recess and the first contact face. The second distance is larger than the diametrical distance of two corners of the object. A slide is located in the cavity and a resilient member is located in the cavity to bias the slide which partially protrudes from the cavity. The slide is restricted from dropping from the cavity by the protrusion. The slide contacts the object.