Polypropylene is used to produce carpet yarns because it is inexpensive. It is difficult to dye and does not wear as well as wool or nylon. Polypropylene is commonly used to construct Berber carpets. In this case, polypropylene is commonly referred to as olefin. Large looped olefin Berber carpets are usually only suited for light domestic use and tend to mat down quickly. Berber carpets with smaller loops tend to be more resilient and retain their new appearance longer than large looped Berber styles. Commercial grade level-loop carpets have very small loops, and commercial grade cut-pile styles are well constructed. When made with polypropylene these styles wear very well, making them very suitable for areas with heavy foot traffic such as offices. Polypropylene carpets are known to have good stain resistance but not against oil based agents. If a stain does set, it can be difficult to clean. Commercial grade carpets can be glued directly to the floor or installed over a 1/4" thick, 8-pound density padding. Outdoor grass carpets are usually made from polypropylene.