Because they're screwed or nailed into the surface (wooden, metal, or concrete), Anti-Slip Bolt-Down Treads, which measure 41/2 x 25 inches, minimize the possibility of coming loose. They are made of aluminum (with 6 or 8 pre-drilled eyelets and rounded corners), over which is either standard or coarse anti-slip tape. Two types are available: flat for walkways and other external areas and L-shaped with a 13/4-inch nosing for steps. Various colors available.