1) Carpet pad 2) Rug pad 3) Underlay 1) Underfelt 2) Underlay 
A material placed under finish coverings, such as flooring, or shingles, to provide a smooth, even surface for applying the finish. 
Asphalt saturated felt used beneath roofing to provide additional protection for the deck.
Factory proportioned combination of portland cement, sand and additives used with or without a latex liquid to level an uneven substrate and to provide a suitable stone floor tile setting surface.
A 
material laid between a subfloor and a finish floor of linoleum, asphalt tile, etc.