Universal Standard for Tubeless (UST) rims are compatible with UST tubeless, tubeless-ready, and standard tires. When used with UST tubeless tires, the tire and rim form an airtight chamber, allowing the tire to be inflated without a tube. When used with tubeless-ready tires, a sealant (sold separately) must be added to the tire to create an airtight seal. For standard tires, a tube can be used. With the tubeless setups, you can run lower tire pressure for optimum traction and control without fear of pinch flats.