In Plan 9, every process draws interacting with a screen file (in fact, it is a whole filesystem, also including keyboard, mouse, control files, ...). What rio, the Plan 9 window manager, does is to "multiplex" this screen file (again, file system) so that each process has its own screen which does not correspond to the physical monitor, but to a window.

An uncommon feature that derives from this design is that you can run a new rio instance inside a rio window. More important, you can just connect to other machines and interact with their windowing filesystems, getting network transparency "for free".