A routing number looks like a single permanent identifier for a bank, and it is not. It identifies a routing endpoint for a particular payment rail, which is why one institution may publish several.
The number addresses an institution, not an account
The nine digits on the bottom left of a check identify the financial institution that holds the account. The account number that follows identifies the account within that institution.
The split matters because the payment system is hierarchical. A transfer is delivered to a bank first, and the bank then applies it to the right account internally.
The final digit is a checksum computed from the preceding eight, which allows software to reject an obviously mistyped number before the payment is submitted anywhere.
Different rails, different numbers
Automated clearing house transfers, wire transfers and paper checks are separate systems with separate processing paths. A bank may route each of them through a different correspondent arrangement.
That produces the common situation where a bank's website lists one routing number for direct deposit and a different one for incoming domestic wires. Neither is wrong.
Using the ACH number for a wire, or the reverse, typically causes the payment to be rejected and returned rather than misdelivered, but the delay can run several days.
Why online banks complicate the picture
Many app-based banking providers are not chartered banks. They hold customer deposits at a partner bank, and the routing number belongs to that partner rather than the brand on the app.
A reader setting up a direct deposit therefore enters a routing number for an institution whose name they may not recognize. That is normal for the arrangement, not an error.
It also means several unrelated apps can share one routing number, and account numbers in that space are issued by the partner bank rather than by the app.
Mergers leave old numbers in circulation
When banks combine, the acquired institution's routing number usually stays valid for a period so existing direct deposits and automatic payments continue to arrive.
Eventually the number is retired and payments addressed to it are returned. This is a frequent cause of a long-running automatic payment failing with no change on the payer's side.
Because the retirement is scheduled rather than sudden, banks notify account holders in advance, and the practical response is to reconfirm the number with any payer sending recurring funds.
What the number does not protect
Routing and account numbers are not secrets in the way a password is. They appear on every paper check a person writes and are shared with every employer.
Protection comes from the rules governing each rail, including the right to dispute unauthorized ACH debits within a defined window. Wires, by contrast, are difficult to reverse once sent.
That asymmetry is the practical reason consumer payment systems steer households toward ACH and card rails, where an error has a documented path back.