The cognitive model underlying Case-Based Reasoning (CBR) has implications for human performance on many tasks, and the technology developed in CBR research can be turned to enhancing performance. For all case-based systems, effective memory retrieval depends on a successful assault on the indexing problem. But demands on indexes may be different, and in some ways, more stringent, for aiding systems than for autonomous probleln-solving systems. This paper report, s on the evolution of an indexing scheme intended to retrieve lessons for architects working on conceptual design of buildings. It illustrates, by example, the process of designing indexing systems, and the demands peculiar to indexes for aiding systems.