GNU Emacs Lisp Reference Manual

35.16: 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 variable tells Lisp programs what window system Emacs is running under. Its value should be a symbol such as x (if Emacs is running under X) or nil (if Emacs is running on an ordinary terminal).

Variable: window-setup-hook

This variable is a normal hook which Emacs runs after loading your .emacs file and the default initialization file (if any), after loading terminal-specific Lisp code, and after running the hook term-setup-hook.

This hook is used for internal purposes: setting up communication with the window system, and creating the initial window. Users should not interfere with it.