Volume 4 is a complete guide to programming with the X Toolkit Intrinsics, the library of C language routines that facilitates the design of user interfaces with reusable components called widgets. It provides concepts and examples that show how to use the various X Toolkit routines. The first few chapters are devoted to using widgets; the remainder of the book covers the more complex task of writing new widgets. Uses the Motif 1.2 widget set in examples and covers X11 Release 5. Volume 4 This book is designed to be used with Volume 5, X Toolkit Intrinsics Reference Manual , which provides reference pages for each of the Xt functions, the widget classes defined by Xt, and the Athena widget set.