Designed specifically to fit straight furniture legs (perpendicular to the floor) that do not have a swivel tip, the 6231 Flexi-Felt® model fits round tubing of of 1 1/4 inch (31mm) and square tubing of 1 inch (25 mm). Its the perfect solution for desks and chairs with straight legs. Like all other Flexi-Felt® products, the 6231 model is made with our wool blend felt. Its 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 its very difficult, almost impossible, for children to remove them. The product is guaranteed for 5 years.