Although the importance of a graphical user interface for Technology CAD has been widely recognized, there is less agreement upon the technical role of the user interface and upon the impact the user interface is granted to have on the rest of the TCAD system. User interface methodology and software engineering aspects are generally underestimated in this context, especially as a graphical user interface is often considered to be just an auxiliary product. This might stem from the fact that from a historical perspective, ``instant'' user interfaces have often (but not always) been quickly added to existing TCAD systems to satisfy impeding user requirements without considering potential long-term impacts on the overall system architecture.
When designed in concert with all other relevant system components (like, e.g., the interactive task level), the user interface can contribute significantly to an advantageous overall system architecture and to the practical utility of the system without affecting existing tools.
The only proven de-facto standard that can be accepted as a stable platform for a Technology CAD user interface is the X Window System. It is available on virtually every modern workstation platform in slightly varying custom implementations, but the so-called Xlib [73] and X Toolkit [74] are well-established standard interfaces which are adopted and strictly obeyed by all vendors. The X Toolkit is an object-oriented subroutine library, designed to simplify the development of X Window applications. The X Toolkit defines methods for creating and using so-called widgets which appear to the user as pop-up windows, scrollbars, text-editing areas, labels, buttons, etc.