window-system-initialization-alist

window-system-initialization-alist is a variable defined in `startup.el'.
Its value is

((x . x-initialize-window-system) (nil . ignore))

Documentation:
Method for window-system initialization.
Window-system startup files should add their own implementation
to this method. The function should take no arguments,
and initialize the window system environment to prepare for
opening the first frame (e.g. open a connection to an X server).