That old saying, “a dollar saved is a dollar earned,” might be a huge understatement. Some one-dollar bills are actually worth over a thousand dollars to collectors. This significant value isn’t from their age but is almost entirely due to their unique serial numbers.

Each U.S. dollar bill has an eight-digit serial number. While you might assume every combination is common, the government printing office often removes the most unusual sequences. This practice makes the unusual ones that enter circulation incredibly rare and sought after by collectors.

There are several specific patterns that make a serial number highly valuable. For instance, a “ladder” is a serial number that runs in perfect order, like 01234567. A “radar” reads the same forwards and backwards, such as 22211222.

Other rare types include the “repeater,” where the first four digits are identical to the last four, like 01570157. An even rarer “super repeater” has two digits alternating all the way through, for example, 01010101.

Some bills are prized for having nearly all identical digits. A “seven in a row” has seven of the same number consecutively, while “seven of a kind” has seven of the same digit that are not all together.

There is also the “binary,” where the serial number is made up of only two different digits, like 11313111. Each of these distinctive patterns can drastically increase a bill’s worth.

Before spending cash, it is wise to quickly check your bills. If you discover a cool pattern, you can research its potential value online. You might just be carrying a small fortune in your wallet.