11th commandment

• 11th Commandment
• eleventh commandment
• Eleventh Commandment

In reference to the biblical Ten Commandments, carrying the implication that the convention is of very high importance (similar to the Ten Commandments).

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11th commandment (plural 11th commandments)

1. (idiomatic) A well-known convention (often the most well-known of a certain field) which supposedly can not or should not be broken.
   • 2016 September 29, Locke, Cathy, “Philosopher Cornel West calls honesty, integrity ‘countercultural’”, in The Sacramento Bee‎, archived from the original on 30 September 2018:

     A “hood,” he said, isn’t necessarily a neighborhood, adding that the same culture is seen on Wall Street, “where the 11th commandment is ‘Thou shalt not get caught.’ ”

   • 2018 September 8, Campbell, Duncan, “After the Hatton Garden heist: 'The actors will profit more than the criminals'”, in The Guardian‎, archived from the original on 30 September 2018:

     Some of the accused did not know Basil’s real identity; the others stuck by the old-school criminal’s 11th commandment, “Thou shalt not grass.”

The "commandment" to which is referred often includes "thou shalt not," or similar phrases, in imitation of the biblical Commandments.