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The digit that catches a slip.
Also known as: Modulus 10 check digit, Mod 10 checksum
The check digit computed by doubling alternate digits, used on payment cards and many identifiers.
Detects transcription errors in an identifier by a weighted sum ending in zero.
Contexts: Public information, Software
Doubles every second digit before summing, which is what lets it catch a transposed adjacent pair as well as a single wrong digit — the two errors people actually make when copying a number by hand.
Contexts: Public information, Software
Catches accidents and not forgery: the algorithm is public, so a number can be constructed to pass it, and passing the check says only that a number is well formed.
Passing the check indicates a well-formed number, not a valid or genuine one.
Contexts: Public information, Software
A final digit appended to a line of numerals, computed so that a weighted sum over the whole string ends in zero.