Its patented design allows the entire felt to remain on the floor regardless of the chair leg angle, greatly increasing the durability of the felt and preventing the accumulation of dust under the felt. Made with our wool blend felt. It's an environmentally friendly alternative to tennis balls and other synthetic products. It has been proven to reduce floor maintenance costs by over 50% as well as reduce furniture movement noise by 90%. Once the unit is installed properly it's very difficult, almost impossible, for children to remove them and the chairs remain stackable.