The Smart Care®-15" TWIN Machine's brushes start with a die-cast polypropylene core for true dimensional stability and balance as well as strength. Bristles are 100% polypropylene which, unlike nylon, will not soften when exposed to moisture. Polypropylene also has a lower melting point to prevent damage to nylon carpet fibers. Depending on the application, different brushes are appropriate. Our brushes are color coded for easy identification. WHITE - For use on delicate/natural fibers such as wool, staple cut or extra deep pile. GRAY - Synthetic fiber cut pile or commercial wool loop pile carpets. BLUE -Commercial grade carpets, including level loop or low cut pile. GREEN -Commercial synthetic fiber - loop pile only. Not for regular use, only for annual restorative cleaning. YELLOW - Severely matted commercial grade low loop pile carpets. Not for regular use, only for severe, "one-time" restoration cleaning. Consult carpet manufacturer and seek approval prior to use. Note: Price is per brush. The machine requires 2 brushes.