Ever noticed how every single Visa card number begins with 4? No? Well, it’s true. And every Amex card begins with either 34 or 37. This isn’t part of some grand, Illuminati conspiracy...it’s just part of the cards’ issuer identification number, or IIN.
Every credit card has an IIN. It’s a six-to-eight-digit long number at the beginning of the card number that identifies who issued it. Not who it was issued to, but who it was issued by.
These numbers exist to protect both the cardholder and the card company against fraud and other identity theft-ish issues; when a transaction goes through, the IIN is used to make sure the card is a valid one, and that its use is authorized.