A carpet is a type of textile floor covering that normally has a backing and an upper layer of pile attached to it. The pile was historically formed of wool, but during the 20th century, synthetic fibres like polypropylene, nylon, or polyester have frequently been employed because they are less expensive than wool. The pile typically comprises of twisted tufts that are heated to preserve their structure. Although the terms “carpet” and “rug” are frequently used interchangeably, rugs are traditionally thought of as being smaller than rooms and freestanding.