A T-line is a simple alternative to a pressure tank, air-trap or any other filling, bleeding, and draining device. It is constructed by plumbing a "Tee" connector (tri-barbed, with a perpendicular secondary flow-pattern, shaped like the letter "T") into an existing tube thus creating an alternate flow-path. However, instead of routing the secondary path back into the system flow, it is simply left open or closed off using an end-barb or other capping device. The tube itself thus can act as a mini-reservoir and allows air-bubbles to travel into it as they are caught into the "Tee" connector, and ultimately travel out of the system (bleeding).

A common use for a T-Line is in a watercooling

Watercooling

Water cooling is a method of heat removal from components and industrial equipment. As opposed to air cooling, water is used as the heat conductor. Water cooling is commonly used for cooling automobile internal combustion engines and large industrial facilities such as steam electric power plants,...

system such as those found in enthusiast PCs. The T-Line is an integral part of a do-it-yourself watercooling system alongside the traditional choice of a reservoir. Major reasons for choosing a T-Line over a standard reservoir are cost, simplicity, and space constraints.

"Fill-Ports" (example "The fillport" by DangerDen) are specifically made with T-Lines in mind, and serve the purpose of capping the T-line in an air-tight and re-openable/re-sealable fashion. They are available anodized in various colors and thus able to be tied in with color-schemes.