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In the beginning, there were no design rules. Experts handling IC design and process development were knowledgeable in both disciplines. As circuit variety and complexity increased, a division of labor became necessary during the early 1970s. Since then, design rules have regulated the design-manufacture interface. These rules represent a compromise between designers, who often desire small, densely packed transistors and circuit elements, and manufacturers, who are concerned with high-yielding processes. Circuit models such as Spice parameters for electrical simulation, and geometrical description formats such as Gerber Data Stream II (GDSII) and Caltech Intermediate Form (CIF), complement design rules and serve as a common language between designers and manufacturers. When a layout - the geometrical shapes that collectively describe a circuit - adheres to all design rules, the fabricated circuit functions according to the design. This article provides a clear view of two complementary approaches to managing the increasingly complex design-manufacturing interface, restricted design rules and model-based design. The author's straightforward discussion is very timely as our industry is moving forward combining both approaches.