While patent scope defined by patent claims provides crucial information on the contribution of underlying inventions to the state of the art, its existing measures do not seem to appropriately capture it, especially with respect to the generality of the inventive concept. This study investigates how significantly the breadth of the first claim can predict the patent’s knowledge impact on subsequent inventions in complex and discrete technologies using the inverse of the first claim length as the indicator. There are two major findings. First, this indicator has very significant predictive power for the knowledge impact of the underlying invention as measured by applicant forward citations, controlling for two existing indicators of patent scope (the number of patent claims and the number of different patent classification codes assigned) in both technology areas. Second, its predictive power for the incidence of top-ranked patents increases in higher quantiles in the complex but not the discrete technology area, unlike the other indicators. This is consistent with an economic model predicting that the knowledge impact of an invention with broad scope has a high variance, depending on the emergence of complementary inventions that enhance the impact of the initial invention.