Internal Program Version: libXt  1.3.1

The low level Xlib library provides functions for interacting with an
X11 server, but does not provide any function for implementing the
graphical objects (widgets) used in GUIs, such as buttons, menus,
etc. The Xt library provides support for creating and using widget
types, but does not provide any specific widget. Specific widgets are
implemented by other libraries using Xt, such as Xaw and Motif.

http://xorg.freedesktop.org