Page "X Window System" Paragraph 45

As X is responsible for keyboard and mouse interaction with graphical desktops, certain keyboard shortcuts have become associated with X. Control-Alt-Backspace typically terminates the currently running X session, while Control-Alt in conjunction with a function key switches to the associated virtual console.

However, this detail is left to the design of an X server implementation and is not universal ; for example, X server implementations for Windows and Macintosh typically do not provide these keyboard shortcuts.