Nails. A nail is a small piece of metal (usually iron) consisting of a slender spike and a head, which is commonly round or flattened. Nails are driven into wood and various other substances in order to hold separate pieces together. They are sometimes distinguished according to the character of their heads; thus the head of the clasp has pointed spurs, that of the clout is round and flat, and the rose has a conical head hammered into triangular facets. The smaller kinds of nail include the tack, a short sharp nail with a flat round head, used in upholstery and saddlery; the brad, used in nailing ceilings and floors, the head of which is merely a slight projection on one side; and the sprig, a sharp taper nail, with no head, used in shoe-making.