I really must do too much shopping online

Earlier today Lincoln needed me to call him with my credit card number.

Lincoln: Is that a Visa?

Me: Nope. A MasterCard. It starts with a 5. MasterCards start with 5, Visa starts with 4, and AmEx cards start with a 3.

Lincoln: But don't AmEx cards have a totally different sequence of numbers?

Me: Sort of. All credit cards have 16 digits but Visa and MasterCard put them as 4, 4, 4, and 4 digits. AmEx does 4, 6, and 5 in a grouping.

(silence)

Me: No, I didn't really know that off the top of my head.

Lincoln: Yeah, I think you did.

TWO NOTES:

1. The reason I knew about the 3, 4, and 5 rules of credit cards is from working at a travel agency during college. Similarly all store department cards will start with any digit other than 3, 4, or 5 unless it's a store card operated through Visa or Mastercard.
2. I was wrong about AmEx having 16 digits. If you add 4, 6, and 5 together you actually get 15 digits. But I was right about the number of digits in each sequence.