The 3-6-3 rule is a slang term that refers to an unofficial practice in the banking industry in the 1950s, 1960s, and 1970s that was the result of non-competitive and simplistic conditions in the industry. The 3-6-3 rule describes how bankers would supposedly give 3% interest on their depositors' accounts, lend the depositors money at 6% interest, and then be playing golf by 3 p.m.