The serial GUI was originally written by MacIntyre [21,20] in K&R C and Motif 1.2 and was later converted to ANSI C and the Motif User Interface Language (UIL) by Levy . The existing user interface requires the proprietary Motif Developer's Libraries which are not available on all of the machines on which the parallel version was intended to be run. Instead, the public-domain packages TCL 7.4b3 and Tk 4.0b3 were used.
The Tool Command Language (TCL)  and its graphical Tool Kit (Tk) allow for the rapid development of MIT X11 Windows-based interfaces as well as provide a rich variety of scripting commands. This made the ideal choice for pLUCAS because its GUI is relatively simple and it primarily manipulates files and strings; tasks which are more difficult in C. Since TCL is a scripting language, changes made to the interface were immediate, without requiring time-consuming recompiling. Developing a new interface in TCL/Tk required a number of days, instead of months, to implement. The primary disadvantage of using TCL/Tk over Motif or the Xt libraries is that it is interpreted and therefore slower and somewhat less robust, but the maintenance is simpler and the time required to learn the package is much less.