The problem of determining the optimal cross‐reference structure for a given index and for a given community of users is discussed. A cross‐reference structure is represented as a graph in which the nodes are index terms and the links are relations between index terms. In order to clarify the concept of ‘level of cross‐referencing’ the characteristics of cross‐reference structures are studied. Some measures of cross‐reference distributions are suggested as a means of comparing the cross‐referencing levels of subject indexes. Types of relations linking the terms of cross‐references in existing indexes and thesauri are examined. The implications of the study for the construction and testing of indexes and thesauri are discussed.