The break key on computer keyboards no longer has a well-defined purpose. Its origins go back to telegraph practices. A standard telegraph key has a built-in knife switch that can be used to short the key's contacts. When the key was not in use, that switch was kept closed, so that a signal was continually sent. If that steady-state signal was interrupted, it indicated one of two things: either the operator was about to start transmitting, or something else had happened to break the connection — such as a physical break in the telegraph line.
As the teletype came into use, a key was created that would temporarily interrupt the communications line, allowing this practice to continue. When this occurred, the receiving teletype mechanism would activate but not print characters (it was typing the non-printing DEL character repeatedly). The noise got the operator's attention.
Time-sharing computers connected to modems could recognize the break because the DEL characters received had "bad parity." This was used to generate an interrupt, allow login, or stop a running program.
On personal computers, the Break key can be used by software in several different ways, such as to switch between multiple login sessions, to terminate a program, or to interrupt a modem connection. On a BBC Micro computer, the Break key generates an interrupt which would normally cause a warm restart of the computer. A cold restart is triggered by typing Ctrl+break. If a DFS ROM is installed, Shift+break will cause the computer to search for and load a file called !boot on the floppy disk in drive 0.
On the Sinclair ZX80 and ZX81 computers, the Break is accessed by pressing Space. On the ZX Spectrum it is accessed by Shift and Space. The Spectrum+ and later computers have a dedicated Break key. It does not trigger an interrupt but will halt any running BASIC program, or terminate the loading or saving of data to cassette tape. An interrupted BASIC program can usually be resumed with the CONTINUE command. The Sinclair QL computer, without a break key, maps the function to Ctrl+Space. On many modern PCs, Pause interrupts screen output by BIOS until another key is pressed. This is effective during boot in text mode and in a DOS box in Windows safe mode with 50 lines. On early keyboards without a Pause key (before the introduction of 101-key keyboards) this function was assigned to Ctrl+NumLock, and this key-combination still works, even on modern PCs with modern keyboards.
In programming, especially old DOS-style BASIC, Pascal and C, Break is used (in conjunction with Ctrl) to stop program execution. In addition to this, Linux and variants, as well as many DOS programs, treat this combination the same as Ctrl+C.
On modern keyboards, the break key is usually labeled Pause/Break. In most Windows environments, the key combination Windows key+Pause brings up the system properties. Compact and notebook keyboards often have the Pause/Break key mapped to a function command: for example Ctrl+Fn+F11.