This investigation reviews the research into journal coverage overlap of the secondary literature indicating the uneven development in this research during the past 30 years and introduces a prototype extension to the traditional definition of overlap. The products of abstracting or indexing services are referred to as secondary sources. Journal coverage overlap (traditional overlap) has been classically defined as the ratio of the number of either journal titles or articles in the intersection of two secondary sources to the number in their union. The extension to the classical definition proposed begins with this traditional definition and proceeds to incorporate the relative sizes of the sources into a matrix of dissimilarity values. Matrices for both the traditional definition and its extension are computed for two universes of secondary source ensembles. These matrices are formed by comparing all possible unions of sources in a universe of secondary sources. Multidimensional scaling analysis is applied to graphically demonstrate this modified concept of coverage overlap and a naive secondary tool selection algorithm is presented. © 1990 John Wiley & Sons, Inc.