A teleprinter (teletypewriter, teletype or TTY) is a now largely obsolete electro-mechanical typewriter which can be used to communicate typed messages from point to point through a simple electrical communications channel.
It had a very limited character set, and poor print quality. The printer was often linked to a punched tape punch and reader allowing the creation of a message stream for offline archival (i.e. save away the paper tape in some box). This was useful for situations in which access to the communication channel was at a premium.
The teleprinter evolved through a series of inventions by a number of engineers, including Royal E. House, David Edwin Hughes, Charles Krum and Emile Baudot.
Teleprinters used the 5-bit Baudot code (also known as IA2) to represent their character set.
The Baudot code was used asychronously with start and stop bits: the asynchronous code design was intimately linked with the start-stop electro-mechanical design of teleprinters. (Early systems had used synchronous codes, but were hard to synchronise mechanically).
A global teleprinter network, called the Telex network, was established in the 1920s, and was used through most of the 20th century for business communications. The main difference from a standard teleprinter is that telex includes a routing network, originally based on pulse-telephone dialing. Telex is still in use for certain applications such as shipping, news, weather reporting and military command. Some business applications are moving to the Internet.
Teleprinters were also used as the first interactive computer terminalss, which had no display. The paper tape function was sometimes used to prepare input for the computer session offline, or to capture computer output.
In computing, especially under Unix and Unix-like operating systems, a teletypewriter is also a name for an external console device, like a user dialing in to the system, or a modem on a serial port. Such devices have the prefix tty, such as /dev/tty13.
For information on the development of telegraphy, including the telex network, see telegraphy.