/usr/X11R6 : X Window System, Version 11 Release 6 (optional)

This hierarchy is reserved for the X Window System, version 11 release 6, and related files.

To simplify matters and make XFree86 more compatible with the X Window System on other systems, the following symbolic links must be present if /usr/X11R6 exists: 

    /usr/bin/X11 -> /usr/X11R6/bin
    /usr/lib/X11 -> /usr/X11R6/lib/X11
    /usr/include/X11 -> /usr/X11R6/include/X11

In general, software must not be installed or managed via the above symbolic links. They are intended for utilization by users only. The difficulty is related to the release version of the X Window System - in transitional periods, it is impossible to know what release of X11 is in use.

Host-specific data in /usr/X11R6/lib/X11 should be interpreted as a demonstration file. Applications requiring information about the current host must reference a configuration file in /etc/X11, which may be linked to a file in /usr/X11R6/lib.