The standard set of widgets in the X world is now the Motif set. Motif offers a large number of widgets, which have seen a lot of development over the last five years. Use of Motif is sometimes a requirement by business, and other widget sets try to conform to Motif in appearance and behavior. Furthermore, many toolkits use Xt-based widgets, so an Xt-compatible interface builder is often useful.
Tm allows the programmer to use Motif widgets instead of Tk widgets from within Tcl programs. This increases programmer choices, and allows comparison of the features of the Tcl Motif and the Tk style of widget programming.
Tm is based on Tk for its style of widget programming, because Tk provides a good model, and to allow Tcl programmers to work with both Tk and Tcl Motif. An alternate style is the WKSH system, a binding of the Korn Shell to the Motif library.