Issuers tack it on so that the whole card number will satisfy a specific mathematical test—the Luhn algorithm. Here’s how the algorithm works: Write out all but the last digit of the card number.

IDEAS Go ahead, write down your passwords The benefits easily outweigh the risks.

Reviews of notable new fiction, nonfiction, and poetry.

My job was to write algorithms. Basically, I was the person who told machines what to do. I was getting significant below market pay for such position, but I was getting paid for mostly doing nothing.