Every American checking account comes with a routing number, and most people treat it as a bank's name written in digits. It is closer to a postal address inside the payment system, assigned and maintained by a central registry.
The number is a directory entry
The routing transit number identifies the institution that will receive an item and settle it. Payment systems use it to look up where an entry should be delivered and which Federal Reserve account is ultimately debited or credited.
Numbers are issued and maintained through a registry administered by the American Bankers Association, with the Federal Reserve relying on that directory for routing. An institution cannot simply choose its own digits.
Because the number points at an institution rather than a branch, the same digits typically serve every branch a bank operates in a given region.
The digits carry structure
The leading digits historically indicated the Federal Reserve district and the processing center associated with the institution. That geographic meaning is a legacy of when paper checks were physically transported for clearing.
The final digit is a check digit, computed from the others using a fixed weighting. Its purpose is to catch transposed or mistyped numbers before an entry is sent anywhere.
A number that fails the check digit test is rejected immediately by validation software, which is why an obvious typo produces an instant error rather than a misdirected payment.
One bank can hold several numbers
Mergers leave acquiring banks holding the routing numbers of institutions they absorbed, sometimes for years while systems are consolidated. Customers keep using the old number until the bank migrates their accounts.
Large banks also maintain distinct numbers for different purposes, most commonly separating wire transfers from ACH entries. The two systems route through different infrastructure with different settlement rules.
This is why a bank's website often lists more than one number and asks which type of transfer is being set up.
The account number does the rest of the work
Routing gets an entry to the correct institution; the account number identifies the customer within it. Account numbering conventions are set by each bank, with no national standard.
That lack of standardization means the payment network cannot verify in advance that an account number is valid. Errors surface only when the receiving bank cannot match the entry and returns it.
Public digits are not a secret credential
Routing numbers are published openly, appear on every check, and are needed by anyone sending a payment. They are not confidential and were never designed to be.
Risk arises when the pair of routing and account number is used to authorize a debit, since that combination can initiate a withdrawal. Authorization rules and return rights, rather than secrecy, are what protect the account.
Treating the numbers as an address, with authorization as the actual lock, describes the system accurately.