Federal Reserve districts & routing-number prefixes
The first two digits of every US routing number tell you which of the twelve Federal Reserve districts settles transactions for the originating bank. Each district maps to a regional Federal Reserve Bank that processes ACH and Fedwire payments for its member institutions.
Reading the first two digits of a routing number
The Federal Reserve assigned the original routing-number geography in 1910, and although banks have merged, charters have moved across district lines, and dozens of consolidations have reshaped the landscape, the prefix-to-district mapping has remained essentially unchanged. A routing number that begins with 01 still settles through the Federal Reserve Bank of Boston; one that begins with 12 still settles through San Francisco. The middle two digits of the prefix narrow the location further to a specific Federal Reserve processing center within the district.
This is invisible to you as a customer, but it explains a few practical differences. ACH transactions originated late in the business day on the East Coast may post a full business day after one originated earlier in the West, because the Fed's Automated Clearing House operates on regional cutoff schedules. Wire transfers settle in real time within Fedwire regardless of district, which is why high-value payments are almost always sent by wire rather than ACH.