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).