Aluminum. To join 2 surfaces of different levels. Carpet-to-tile, carpet-to-hardwood, low pile to high pile carpet. Screw nails included. Carpet metal installation on concrete surface suggestion: Drill a hole in the concrete, insert a 3/16″ x 1-1/4″ or 1/4″ x 1-1/4″ wood dowel. Use the screw nails provided with carpet metals, insert into dowel, expanding the dowel to lock in place.