The X Toolkit consists of the intrinsics and the Athena widgets. The Intrinsics provide the base mechanisms for simplifying the design of user interfaces. The first part of the book is about the Intrinsics - C Language interface. This is intended for the application programmer using widgets built with the Intrinsics and for the widget programmer building widgets using the Intrinsics. The second part is for users of the Athena widget set, a cohesive set of predefined widgets built using the Intrinsics.