11th commandment

A well-known or widely accepted practice or axiom in a particular setting or situation. Because the phrase alludes to the Biblical 10 Commandments, 11th commandments sometimes also start with "Thou shalt." Fostering strong relationships with clients is the 11th commandment around here—after all, they're the ones that keep us in business. "Thou shalt clean thy room" was the 11th commandment in my parents' house when I was growing up. Joe will be here on time, I'm sure—keeping appointments is like the 11th commandment for him.