Computer aided design (CAD) software delivered by vendors is seldom complete. Customizing and/or developing new software is inevitable. A recent survey indicates that most companies purchase or write custom computer aided design software tailored to their specific needs. Design of such software requires the development of modeling tools for the creation of CAD geometry. This research investigates the capabilities of the proposed graphics standard, PHIGS plus, as a geometric modeling interface in CAD that off-loads many basic modeling functions to PHIGS. Specifically, how the B-spline modeling capabilities of PHIGS plus can be enhanced through the use of a new modeling interface prototype is examined. This prototype consists of a proposed graPHIGS modeling toolbox based entirely on PHIGS plus concepts. The prototype provides functionality to easily create B-spline curves and surface geometric entities based on data points and continuity constraints. Functionality is also included to easily perform modeling operations on B-spline surfaces such as intersection and trimming. / Master of Science