A tee valve is a type of joint used in pipe fitting, and it gets its name from its shape. It is needed when the water flow between pipes needs to be split in two different directions or combined into a single pipe. This depends on the directional flow of the water. The section of the valve that forms the top of the T shape is known as the inlet/outlet, and this is most commonly the same diameter as the length of pipe being fitted to the tee valve.
Tee valves are one of the components needed to build what is called a T-Line in plumbing projects. A T-Line consists of the needed length of pipe, the tee valve(s) and an end cap that directs the water flow from the lines of the tee valves back into the main water system. Tee valves are available in the same materials as many pipes: copper, PVC, iron and others.
A T-Line setup is often used for water-cooling systems as an alternative to a reservoir system. The reason for this choice is that a T-Line is much more economical in terms of cost and use of space and is easier to construct. It is the setup of choice for DIY plumbing and pipe-fitting projects.