A carpet is a thick floor covering of soft, textile material which is laid over a floor or a staircase as a finishing floor layer. A carpet is typically consisting of an upper layer of pile attached to a backing. The pile was traditionally made from wool, but, since the 20th century, synthetic fibers such as polypropylene, nylon or polyester are often used, as these fibers are less expensive than wool. The most important features affecting quality performance are pile fiber, pile yarn, pile height, pile density, carpet surface structure (cut pile or loop pile), carpet construction, etc.