Shell Shutter Dog Set, hand cast in iron with a black base primer coat. 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.