In the plumbing trade with a cap fitting or cap is meant a type of pipe fitting, often liquid or gas tight, which covers the end of a pipe. A cap has a similar function to a plug. cap fittings are designed to attach over the outside of a tube end to dead-end the flow. A plug is inserted into the end of hose or tubing to dead-end the flow. Attachment techniques include welding, push-fit, brazing, adhesives, and threading.In plumbing systems that use threads the cap has female threads.