In those days, most banks used a system that allowed an employee to identify himself to a fellow employee at another branch by providing a code that changed every day. For example, Bank of America used five daily codes, labeled "A," "B," "C," "D," and "E," each of which was assigned a different four-digit number. An employee calling another branch for information would be challenged to give the correct number for code A or code B or whatever. This was the banking industry’s idea of foolproof security.