Work is described on patent citation networks, a novel technique for displaying the history of technological subjects and their key turning points. The method accurately identifies the key patents in a subject field, and if a subject docs not have a definite starting point, this is reflected in the patent network. The networks are less satisfactory if the key patent took a long time to appear in print. Subjects studied were: semi‐synthetic penicillins, tobacco substitutes, electrophotography, Ziegler‐Natta catalysis, and hovercraft. Possible uses of the technique are examined.

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