Record units, each addressable by a record unit address, have keyfields with keyfield bits arranged to be presented in descending order of significance. The record unit addresses are separated into first and second address groups, comprising all addresses corresponding, respectively, to record units having a most significant keyfield bit of 0 and 1. The first and second address groups are each similarly subdivided into two successive address sub-groups in dependence on the next most significant keyfield bit of each record unit. The separating process of each successive sub-groups of addresses is continued under control of equally weighted bits from each record unit until all keyfield bits have been utilized. A system of indicator numbers are assigned to each record unit address and are modified during each successive sub-grouping to reflect from which of the immediately preceding sub-groups the address is derived.