Older Unix GUI toolkit, and the best looking one (according to some developers) developed on top of the Xt intrinsics.  A real PITA to program for, but that's Xt (and, for that matter, C) for you.

IMO, if you've got a choice develop for something else ([Tk], [GTK], [Qt], anything!) rather than Motif or the other Xt-based toolkits (the most widely known one of which is the [Athena Widgets], whose main use these days seems to be [xterm]...)

historic migration to Tk reflections http://pcbunn.cithep.caltech.edu/jjb/kuip2tcl.html

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