A little over 50 years ago, John Shepherd-Barron was taking a bath and thinking about a machine that could dispense cash instead of chocolate bars when, he claims, he came up with the idea for the ATM. Realizing he needed a way for the machine to safely identify the user, he then suggested the PIN.
According to the BBC profile, "The Man Who Invented the Cash Machine, " with his old army number in mind, he originally settled on having a six-figure personal identification code. But his wife, Caroline, rejected the idea. "Over the kitchen table," he told BBC with a laugh, "she said she could only remember four figures, so because of her, four figures became the world standard."
Caroline's forgetfulness has made millions of lives more convenient, but it has also exposed users to some security issues. Instead of one million possible PIN number combinations, hackers only need to consider 10,000. This substantial difference is what inspired Apple to pivot to the six-digit passcode when they released iOS9 for the iPhone.
But 10,000 combinations, it seems, is still plenty. Most ATMs allow you 15 attempts to type in your PIN, so the chance of someone guessing your number correctly is low, assuming your code isn't too obvious.
Unfortunately, users "exhibit a staggering lack of imagination and select very predictable numbers," reports The Guardian. Three very simple combinations — 1234, 1111 and 0000 — account for 18.6 percent of all PINs in use.
Shepherd-Barron's instincts haven't always been trustworthy. Once he tried to change the fish farming industry by inventing a machine that played killer whale sounds to ward off seals. It actually attracted more seals.
But both the ATM and the PIN have been a hit. There are more than three million ATMs worldwide, including one in Antarctica.
It was not until the couple was vacationing in Chiang Mai, Thailand, Shepherd-Barron told the BBC, and they saw a farmer on a bullock cart remove his wide-brimmed hat to use a cash machine, that he realized he had changed the world.
Like this story? Like CNBC Make It on Facebook.