This is specifically a butt gauge, made for laying out the mortises for a butt hinge, both on the door and on the frame. You set one arm for the proper distance back from the edge of the door, and the other arm for the proper distance from the edge of the frame. I have one Stanley and one Miller's Falls like yours, though yours has the wheels while mine both have cutters to mark out. Both of mine have a third cutter on one of the arms to gauge the depth of the mortise necessary for the hinge to be flush with the jamb and the door.