Most of us don’t think twice about a $1 bill. We crumple it, toss it in a tip jar, or forget it in our pockets. But what if that plain piece of paper was actually worth a small fortune?

Collectors and currency experts are on the lookout for rare serial numbers — and one specific type could make you thousands of dollars richer.

So, what makes a $1 bill valuable?

The serial number is the key. It’s that unique set of numbers and letters printed on the front of the bill. Some combinations are considered “fancy serial numbers”, and they’re in high demand among collectors.

Here are a few to look for:

• Low serial numbers: like 00000001 to 00000100
• Repeater numbers: like 12344321 or 88888888
• Ladder numbers: like 12345678
• Radar bills: numbers that read the same forward and backward, like 01133110
• Birthday serials: numbers that look like a date, e.g., 07041876
• Star notes: bills with a star (*) at the end of the serial number

But the rarest and most valuable $1 bill to look for is called a “binary repeater” — a bill with only two numbers repeating, such as 01010101 or 29292929. These are exceptionally rare and can sell for up to $5,000 or more, depending on condition.