Hi Mike,
This can get complicated, as different museums employ different numbering systems as appropriate for their particular collections (or the same museum changes its numbering system over time). I think Becca answered it very succinctly – we also use this trinomial system at the SHSND, although it has changed over time (the first number has gone from simple sequential numbering to 2-digit years to 4-digit years). In any case, the catalog number should be an extension of the accession number. I would recommend Museum Registration Methods put out by the AAM Press for more detail on numbering systems, collections management, etc.
