All mainstream credit card numbers obey a mathematical trick designed to catch the most common typos. It’s called the Luhn ...

You’re at the checkout screen after an online shopping spree, ready to enter your credit card number. You type it in and ...

Come the start of the 1920s, though, this would change, and four digits became the standard. Phone numbers that hadn't already adopted this format would add zeroes to fill in the blanks.