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On-line circuit design involves interaction between a designer who initiates circuit modifications on the basis of circuit performance, and a digital computer which calculates the performance resulting from such modifications. Besides network analysis, the flexibility of an on-line utility makes possible a variety of other services such as on-line definition and modelling of new circuit elements, while graphical input and display devices facilitate man-machine communication, requiring no programming knowledge on the part of the user. Compared to conventional design or batch-computer design, the on-line approach includes advantages such as savings in design time and cost, and progressive accumulation of diverse and accurate knowledge about expected circuit behavior, before actual construction is undertaken. This paper discusses a family of on-line nonlinear circuit-design systems known as CIRCAL, in terms of features visible to a user, internal structure, supporting theory, experience, and future plans.