Chimera is a cross-platform application (Windows and UNIX). To save programmer time and effort, we chose the Tkinter library provided with Python [6] as the base user interface library. Tkinter is a wrapper around the tk [7] library, which provides a cross-platform GUI with native look and feel on the Windows, UNIX and Macintosh platforms. On top of the Tkinter library, we use Pmw [8] and Tix [9], which provide user interface components which can be combined to generate sophisticated dialogs.