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### 37.24 Window Systems

Emacs works with several window systems, most notably the X Window System. Both Emacs and X use the term “window”, but use it differently. An Emacs frame is a single window as far as X is concerned; the individual Emacs windows are not known to X at all.

Variable: window-system

  This terminal-local variable tells Lisp programs what window system Emacs is using for displaying the frame. The possible values are

  `x`

    Emacs is displaying the frame using X.

  `w32`

    Emacs is displaying the frame using native MS-Windows GUI.

  `ns`

    Emacs is displaying the frame using the Nextstep interface (used on GNUstep and Mac OS X).

  `pc`

    Emacs is displaying the frame using MS-DOS direct screen writes.

  `nil`

    Emacs is displaying the frame on a character-based terminal.

Variable: initial-window-system

  This variable holds the value of `window-system` used for the first frame created by Emacs during startup. (When Emacs is invoked with the `--daemon` option, it does not create any initial frames, so `initial-window-system` is `nil`, except on MS-Windows, where it is still `w32`. See daemon in The GNU Emacs Manual.)

Function: window-system &optional frame

  This function returns a symbol whose name tells what window system is used for displaying frame (which defaults to the currently selected frame). The list of possible symbols it returns is the same one documented for the variable `window-system` above.

Do not use `window-system` and `initial-window-system` as predicates or boolean flag variables, if you want to write code that works differently on text terminals and graphic displays. That is because `window-system` is not a good indicator of Emacs capabilities on a given display type. Instead, use `display-graphic-p` or any of the other `display-*-p` predicates described in Display Feature Testing.

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