This paper describes the development, through three versions, of a computer‐based subject‐index production scheme designed as a complement to a system producing MARC‐based catalogues for a library cooperative. The scheme provides an individual subject index to each member of the cooperative and a union subject index to the whole, or any part, of the group. The major advantages of the scheme are simplicity and attention to practical detail. The disadvantages are an eighty‐character limitation on record length and complete dependance on manually produced records. The scheme is designed to handle records carrying Dewey classification numbers.