This classic "S" style shutterdog is solid cast iron. This shutter dog measures 6" long with a 4-1/2" lag screw that can be used for wood or brick. This shutter dog is reversible so it can be mounted right or left hand. Each design is a careful reproduction of an antique design with excellent detail. In engineering, a dog is a tool or part of a tool that prevents movement or imparts movement by offering physical obstruction or engagement of some kind. It may hold another object in place by blocking it, clamping it, or otherwise obstructing its movement. So a "Shutter Dog" is a stay that holds a shutter in place. These are traditionally used on the exterior of a house to hold a shutter open. They are historically found in a variety of shapes including grapes, shells, "S" shapes, and "rat-tails" among others.