You write a cheque to X.
X hands it to their bank A.
A asks your bank B for the amount on the cheque.
B asks A for the cheque to check its valid.
B sends money to A.
Done. Which is why I thought they take days to be processed.
Used to work that way. Not anymore. Routing number and account and amount are sent electronically to a check clearinghouse and the money is transferred sometimes the same day or even close to real-time. The paper itself gets scanned, and the original shredded.
You write a cheque to X. X hands it to their bank A. A asks your bank B for the amount on the cheque. B asks A for the cheque to check its valid. B sends money to A. Done. Which is why I thought they take days to be processed.