PDMSis a specification-driven, 3-dimensional (3D) modeling system, consisting of a single relational database management program and several separate and distinct modules, each performing a unique function. Unlike many CADD systems, many of which began with a graphics package and later added database capabilities, PDMS' designers approached plant design as a true data management problem. Their solution was to establish a database core and provide methods to display the contents graphically. This approach eliminated the problem of synchronizing the graphic and data components of a graphics-based CADD system.