Why ATM machine PIN numbers have 4 digits:

One by-product of inventing the first cash machine was the concept of the Pin number.

Mr Shepherd-Barron came up with the idea when he realised that he could remember his six-figure army number. But he decided to check that with his wife, Caroline.

"Over the kitchen table, she said she could only remember four figures, so because of her, four figures became the world standard," he laughs.

(Via Bruce Schneier.)