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The break key found on many modern keyboards has a history that can be traced back to telegraphs. This early use of a break key was actually a switch that could be used to bridge the contacts within a telegraph machine and send a constant signal when it was not being used. Later examples of break keys existed in early teletype machines and computers that used a time-sharing scheme. Modern personal computers have also included break keys in many keyboard designs, and they have had a variety of functions. Most modern keyboards have a combination pause/break key that may halt text output or exhibit different behaviors, depending on the program in use.
Telegraphs allowed for long distance communication by electrically connecting two machines via a wire. These devices were operated by depressing a button to successively create and break an electrical connection in a specific pattern that could be interpreted to represent numbers and letters. When not in use, the signal was typically sent in a continuous manner to indicate that the transmission line was in working order. Prior to transmitting a message, a break switch or key could be activated to interrupt the connection.
Later teletype machines also included a break key which could be used to alert the operator that a message would soon be transmitted. The break key in this circumstance typically transmitted a special, non-printing character that could activate the remote machine prior to an actual message being sent. This same key and non-printing character also had a use in time-sharing computers, since it could often be used to interrupt, or break, a modem connection.
Some early microcomputers also had break keys, though the functionality was typically different. These break keys were sometimes used to initiate warm system reboots or cold restarts when used in conjunction with other keys. Microcomputers that lacked break keys would sometimes be able to simulate their functionality with a different key combination.
Modern computers often have a combined pause/break key, though its function is not universal across operating systems (OSes) or even programs. Some computers use their pause/break key to halt text displays in disk operating system (DOS) or other similar environments. One common use of this is to press pause/break during boot in order to read the text output from the basic input/output system (BIOS). It may also be useful when operating a DOS box or terminal window that has a limited number of display lines and no scrolling option. In other circumstances, the key is used to interrupt a program or, in combination with another key, to display information.