[mcpherrinm]

The mechanical copiers have never been the best way to copy a key.

Each key manufacturer has a fixed set of depths to cut each position on the key at, which you can represent as a single digit. Combined for the whole key and you can talk about the data encoded into the piece of metal as a string of digits.

Telling you my apartment key is a kwikset KW1 with bitting 64265 is enough to cut a new key.

[darkmighty]

I suppose having a discrete set prevents error propagation also, avoiding copies drifting into unusability.