In 1973, Small published a paper that was as groundbreaking in its own way as Garfield’s 1955 paper introducing citation indexing for science. This paper, “Co-citation in the scientific literature: a new measure of relationship between two documents,” introduced A new era in describing the specialty structure of science11. Small measured the similarity of two documents in terms of the number of times they were cited together, in other words their co-citation frequency. He illustrated his method of analysis with an example from recent papers in the literature of particle physics. Having found that such co-citation patterns indicated “the notion of subject similarity” and “the association or co- occurrence of ideas,” he suggested that frequently cited papers, reflecting key concepts, methods, or experiments, could beused as a starting point for a co-citation analysis as an objective way to reveal the social and intellectual, or the socio- cognitive, structure of a specialty area. [...] Small found co-citation analysis pointed to the specialty as the natural organizational unit of research, rather than traditionally defined and larger fields. Small also saw the potential for co- citation analysis to make, by analogy, movies and not merely snapshots. “The pattern of linkages among key papers establishes a structure or map for the specialty which may then be observed to change through time,” he stated. “Through the study of these changing structures, co-citation provides a tool for monitoring the development of scientific fields, and for assessing the degree of interrelationship among specialties.”