What does the letters mean in us currency serial numbers?

In U.S. currency serial numbers, the letters represent the Federal Reserve Bank that issued the note. There are 12 Federal Reserve Banks, each designated by a specific letter from A to L. For example, 'A' corresponds to the Federal Reserve Bank of Boston, while 'B' corresponds to the Federal Reserve Bank of New York, and so on. The letter helps identify the geographic origin of the currency.