Bank Card PIN: Why Four Digits?

Do you know why a bank card PIN consists of exactly four digits? The story behind today’s familiar standard is connected to John Shepherd-Barron, the inventor of the first ATM, and his wife, Caroline. Shepherd-Barron originally wanted to use six-digit codes, believing that six digits would provide more possible combinations and therefore better security. However, his wife Caroline said she would not be able to remember more than four digits. The inventor listened to her argument — and that is how the four-digit PIN was born, eventually becoming a global standard.

The first ATM operated by Barclays, known as Barclaycash, went into service in London in June 1967. However, calling it a modern ATM would be quite a stretch. Bank plastic cards as we know them today did not yet exist, so customers needed a special paper voucher that was inserted into the machine. And this is where the story gets particularly interesting: the vouchers contained a small amount of radioactive carbon-14. The machine read the special marking, after which the customer could receive cash.

The system was designed to automate cash withdrawals outside a bank branch. Shepherd-Barron worked on the idea of a machine that would allow people to withdraw money around the clock, because banks closed — while people’s need for cash, for some mysterious reason, did not. Initially, the machine dispensed a fixed amount of £10. The customer inserted a special voucher, the machine checked it and then released the money.

Shepherd-Barron was not only the inventor of the ATM. In his later years, he lived on a remote farm in Scotland and raised salmon. And this is where his life unexpectedly collided with another problem that had absolutely nothing to do with banking technology. In an interview, he complained about seals that regularly attacked his fish farm and stole his salmon: “They’re clever scoundrels,” the inventor said angrily.

It makes for a rather amusing story: the man who invented a way to dispense money safely without a cashier eventually encountered an opponent against whom no PIN code could help. The seals apparently preferred a more traditional payment system — no cards, no ATMs and no fees.

Today, a four-digit PIN seems so obvious that few people ever think about where it came from. But the next time you enter four digits at an ATM, remember that behind this simple code lies a very specific family story. If Caroline Shepherd-Barron had been comfortable remembering six digits, perhaps we would all be using six-digit PINs today. And perhaps every time we entered one, we would curse her husband for being so unnecessarily forward-thinking.