A T1 line is a type of telephone line that can handle 24 speech lines for phone service or 1.544 Megabytes of total capacity for Internet connections. T1 lines can travel over coax or fiber optic connection in addition to the more common twisted pair copper wire, which is akin to telephone wire. DS1 (Digital Signal Level 1) lines are another name for T1 lines.

In order to digitally transmit numerous telephone lines over a single link, known as a trunk line, between telephone switching hubs, telecoms first introduced T1 lines in the 1960s. Later, telecoms provided companies with specialized T1 lines that could deliver up to 24 phone lines over a single link. Finally, T1 connections gave those companies consistent, dependable access to the Internet. Users that required more capacity could buy a connected T1 connection, providing twice the speed of a single T1.

T1 connections lost favor as more high-speed internet options became accessible.

As part of the overall closure of copper cable networks, some Providers that had provided T1 lines for years are now ceasing to do so.