This is one method of fastening a fireplace mantel to the wall behind the mantel. This is certainly not the only method but it is useful in certain situations. The advantage of this method is that the "cleats" are hidden as are any other screws or mechanical fasteners. The mantel is attached by sliding it over the cleat. A recess, however, is required at he mantle rear to attach the cleat to - something not always possible. But if you are making the mantle from scratch this should not be a problem. To see the original article click here: Mantle Fastener