Data Description Languages (DDLs) usually are discussed in terms of business data processing applications. This paper describes the importance of DDLs in computer-aided design (CAD). Users of CAD systems are compared with users of business data processing systems, and are shown to have radically different skills, view data in different ways, and perform different operations upon data. Users of CAD systems are concerned not so much with frequent update or casual interrogation as with powerful and flexible representation of interconnections and mathematical constraints among components. The implications of CAD requirements for the relational and network models are discussed.