A T1 line refers to a specific type of copper or fiber optic telephone line that can carry more data than traditional telephone lines. The T-carrier line, as it is sometimes called, was developed by AT&T Bell Labs for North America and Japan.

Twisted copper telephone lines have been the standard for decades, transmitting voice and data via analog signals. Today that standard is slowly being upgraded to fiber optic lines, (lines made of bundled glass fibers), but most T1 lines are still made of twisted copper. The T1 line creates a "pipe" capable of blowing through larger datastreams.

While standard telephone lines can transfer data and voice at a rate of about 30,000 bits per second (30 kbps) using a dial-up modem, a T1 line can transmit 1.544 megabits per second, or can be used to transmit 24 digitized voice channels. Hence it can be used for phone service in a commercial building, for instance, or for data transfer on a network, providing service up to 60 times faster than a traditional modem. Businesses with more than 8 phone lines may save money by getting a dedicated T1 line to loop to the place of business from the local phone company office. This can reduce telecom charges and provide high-speed Internet access at the same time. The business is charged for one T1 line, rather than eight (or more) separate telephone lines. Pricing for the T1 line will depend on the distance of the loop, or the mileage between the telcom and the place of business.

The cost of a T1 line can be expensive, but prices are dropping as demand grows. Internet service providers (ISPs) will lease T1 lines to provide service to their network of clients. Other multiplexed fiber optic lines include T2 and T3 lines, which can transfer up to 44.736 megabits per second. A T3 line is equal to having 28 T1 lines, and is used by larger businesses with higher data and voice demands. More advanced T-standards also exist, though are not in high use.

The European Union uses a different, incompatible type of carrier line, called the E1 and E3. The E2 line is also available but less common.

Generally speaking, a T1 line is not cost-effective for individual or residential use. In this case DSL or cable service is a better choice. Small businesses and government agencies — particularly those using a PBX — will benefit most from a dedicated T1 line.