X Window System

The X Window System is an open source (mostly MIT licensed) framework for building graphical user interfaces. It provides the abstraction layer between the hardware and the GUI toolkits, allowing a variety of user interfaces to be developed on top of it. It was included in most Unix variants, and until recently, was the primary GUI layer on Linux systems as well, though newer options such as Wayland and Mir are being developed now to replace X.

X Version 11, aka X11, was first released in September 1987, and has maintained compatibility of the network protocol and client API since then.

Releases from X11R1 in 1987 through X11R6.9 in 2005 consisted of the entire window system in one unified release, built via the Imake build tool.

Starting with X11R7.0 in 2005, X11 was broken up into individual modules which are released individually, each configured & built with the GNU autotools suite (autoconf, automake, libtool, pkg-config). Rollup “katamari” releases of all the latest individual modules are still released occasionally, most recently as X11R7.7 in 2012, but most distros now rely on just tracking the individual package releases instead. Releases of modules, katamaris, and security fixes are announced on the xorg-announce mailing list.