Back in the day, when processing payments for online transactions cost the earth and pre-auth was but a twinkle in the eye of the bank API developers, knowledge

Back in the day, when processing payments for online transactions cost the earth and pre-auth was but a twinkle in the eye of the bank API developers, knowledge of the Luhn algorithm allowed one to test services via their free trials without having to give up one's credit card number. These days it's only useful as a pre-pre-auth sanity check.