You know the expression “a dollar saved is a dollar earned”? Turns out that a dollar saved could actually be worth a lot more, like, hundreds of dollars more.

Coins aren’t the only type of currency that collectors will pay big bucks for. There are $1 bills that are selling for over $1000. No joke!

What makes these dollar bills so valuable? The rarity of their serial numbers is key.

Each dollar bill in the United States has 8 digits. You’d think that each combination of numbers would be equally likely, but that’s not the case. The Bureau of Engraving & Printing often removes the weirder number combinations which ends up making the ones that get through more valuable.

Here’s a video that gives a great preview of which serial numbers are especially rare:

Go get your wallet, and see if any of these types of rare serial numbers are on any of your dollar bills:

1. Ladder

   A ladder is when the numbers are in order; for example, 01234567.

2. Radar

   A radar serial number means that the first four digits are the reverse of the last 4 digits; for example, 22211222.

3. Super Radar

   This is really the same as a radar serial number but even rarer because the middle six digits are all the same number; for example, 23333332.

4. Repeater

   A repeater serial number is like it sounds; the numbers repeat; for example, 01570157.

5. Super Repeater

   A super repeater is when two digits are repeated all the way through the serial number; for example, 01010101

6. Radar Repeater

   Like it sounds, a radar repeater serial number is a radar and a repeater; for example, 12211221.

7. Double Quad

   A double quad is when there are only 2 digits in the serial number with the first 4 being one digit and the last 4 being another digit; for example, 00003333.

8. Seven in a Row

   A seven in a row serial number is when 7 of the digits in the number are the same; for example, 11111115

9. Seven of a Kind

   When there are 7 digits in the serial number that are the same but they aren’t all together, it’s known as seven of a kind; for example, 11115111.

10. Binary

    A binary serial number is when there are only two digits in the serial number, but they’re not all together and there are less than 7 of either number; for example, 11313111

Before paying for anything with cash, check the serial numbers on your dollar bills and see if they they include any of the patterns listed above. If you find a cool serial number, do a quick search at coolserialnumbers.com to see how much your dollar bill is worth.

Do you have any rare serial numbers in your wallet? If you don’t collect rare dollars, what kind of collection do you have?