In many GUI environments, including Microsoft Windows and most desktop environments based on the X Window System, and in applications such as word processing software running in those environments, control-C can be used to copy highlighted text to the clipboard. Control-C was one of a handful of keyboard sequences chosen by the program designers at Xerox PARC to control text editing. Presumably these particular keystrokes were chosen because of their location on a standard QWERTY keyboard, since the Z (undo), X (cut), C (copy), and V (paste) keys are located together at the left end of the bottom row of the standard QWERTY keyboard. The equivalent Mac OS key combination on Apple computers is Command-C (or Apple-C).
Under most Unix-like systems control-C is used to terminate the process currently in the foreground in a command shell by sending it a SIGINT. This system is usually preserved even in graphical terminal emulators; in this case, different keystrokes are usually chosen for copying the selection, such as control-insert or control-shift-C.