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.
This terminal-local variable tells Lisp programs what window system Emacs is using for displaying the frame. The possible values are
Emacs is displaying the frame using X.
Emacs is displaying the frame using native MS-Windows GUI.
Emacs is displaying the frame using the Nextstep interface (used on GNUstep and macOS).
Emacs is displaying the frame using MS-DOS direct screen writes.
Emacs is displaying the frame on a character-based terminal.
This variable holds the value of
window-system used for the first frame created by Emacs during startup. (When Emacs is invoked as a daemon, it does not create any initial frames, so
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
Do not use
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.