These push-to-connect fittings are offered in Copper Tube Sizes (CTS) and are made from engineered plastic with EPDM O-Rings or compliant brass. These fittings can be used with Copper, PEX and CPVC Pipe. The Twist-to-Lock feature, allows you to push the pipe in, and then twist a locking collar to remove the possibility of accidental pipe release. When using PEX tubing, inserts are required for connections in all heating applications and are recommended for standard plumbing applications. For UV resistance, use the 99XX series of black fittings. Do not use for gas, fuel oil or compressed air applications.