If a retailer asks for the last four digits of your credit card, but you’ve used Apple Pay, you might be out of luck if you use the actual digits off your plastic rectangle.

Every time you give a retailer or waiter your credit or debit card to pay for goods or services, the actual account number is there for them to steal. When you use Apple Pay, however, those numbers are hidden behind a unique “Device Account Number,” which is assigned, encrypted, and stored on a dedicated chip on your iPhone or Apple Watch. They don’t even get stored on Apple’s servers.

Finding that Device number, though, can be tricky. Here’s how.

If you have more than one credit card, chances are you’ll want to put them all into Apple Pay so you can use any of them when the mood strikes, or your card balance dictates.

If you’ve got an Apple Watch, you’ll need to add them to the Watch via a separate process than the way you added them to the iPhone.

Once you’ve added more than one card, though, you might want to change the default Apple Pay card. Here’s how to do just that.