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X Toolkit Intrinsics is a library that implements an API to facilitate the development of programs with a graphical user interface for the X Window System. It can be used in the C or C++ languages.
The low-level library Xlib is the client-side implementation of the X11 protocol. It communicates with an X server, but does not provide any function for implementing graphical control elements , such as buttons or menus.
The Xt library provides support for creating widget types, but does not provide any itself. A programmer could use the Xt library to create and use a new type of widget. Xt implemented some object oriented concepts, such as inheritance , events, and callbacks.
Since the graphical user interface of applications typically requires a number of widget types, most developers are reluctant to write their own, and instead prefer to use libraries such as Xaw, Motif and OLIT that provide a set of widgets.