A class of index structures for use in a virtual memory environment is described. Design alternatives within this class of index structures are analyzed. These alternatives include a choice of search strategy, whether or not pages in the index are stuctured, and whether or not keys are compressed. The average cost of retrieving entries from these indexes is expressed as a weighted sum of the cost of a basis key comparison and the cost of crossing a page boundary in the index structure. Formulas for the retrieval costs possible combinations of design alternatives are given. These are used in numerical case studies which compare the retrieval costs of the alternatives. Qualitative comparisons of the maintenance costs (insertion, deletion, reorganization) of the design alternatives are also included. © 1977, ACM. All rights reserved.