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. Volume 4 is available in two editions. The Standard Edition uses Athena widgets in examples for X11 Release 5 to demonstrate how to use existing widgets, while still providing a good introduction to programming with any widget set based on Xt. Volume 4 includes: Introduction to the X Window System. Building applications with widgets. Constructing a bitmap editor with widgets. An overview of each widget in the widget set. Basic widget methods. Events, translations, and accelerators. Event handlers, timeouts, and work procedures. Resource management and type conversion. Selections and window manager interaction. Geometry management. Menus, gadgets, and cascaded pop-ups. Miscellaneous techniques. Comparison of Athena, OSF/Motif, and AT&T OPEN LOOK widgets. 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.