Debit memos from a bank reduce a depositor's cash balance, and credit memos increase the balance. This seems backwards from the normal debit-and-credit rules. What is the explanation? From a banks perspective, customers deposits are liabilities. Banks are required to guarantee the safety of all customer deposits. Bank accounting practices were created in 1943, before the debit-and-credit rules were adopted. State banking regulations require banks to use different accounting practices.