Made of polytetrafluoroethylene (PTFE, better known as Teflon®), capped seals (also called slipper seals) are used in combination with standard O-rings. Because the PTFE cap drastically reduces break-out friction, capped seals are often used in applications where machinery undergoes prolonged idle periods. As added benefits, PTFE capped seals have outstanding chemical resistance and can operate across a wide temperature range (-300° to +450° F). Capped seals are available in two configurations: one which seals on the I.D. (the “kin” configuration) and one which seals on the O.D. (the “kex” configuration, see Figure 142).