A teletypewriter (TTY; also called a teletype or teleprinter) is a device that sends a typed message to another place. A teletypewriter has a typewriter keyboard, a local printer (so the user can see what has been typed) and a transmitter. Messages can be sent over wires or radio waves. Teletypes were used mostly in the early to mid-20th century. They were developed to improve telegraphs, some of which also used keyboards, though not typewriter keyboards.