X11 was written at MIT in the early 1980s and made available under an open source license similar to Berkeley. There had been several previous X versions but X11 became widely adopted and the numbering stopped there. There were alternative windowing systems, such as Sun’s Network-extensible Windowing System (NeWS), but X11 won out and became the de facto standard for windowing on UNIX and similar systems. X11 was innovative in important ways. Unlike its rivals, the Windows and Mac GUI, it is portable across many systems and is not hooked to a particular operating system. It separates the display component (or X server) from the client application, so that applications can be operated across a network. It sup- ports virtual desktops, so you can scroll across a much larger space than the physical display. And it is much more customizable than other systems.