I recently got an AmEx credit card and it seems a bit different than all the other cards I'm used to.
- Instead of 4 groups of 4 digits, the card number is 15 grouped in 4, 6, and 5. I'm not sure if this is normal or if there's a digit missing that I'm supposed to memorize or get from somewhere else.
- There's a 4-digit code on the front of the card and a 3-digit code on the back which aren't related (the 3-digit one isn't part of the 4-digit one). I'm not sure which one of these is my verification code.
Is there a reason for these to be different?