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1.
The Cardinal Rule a substantial rule that is in place in a certain situation or organization. And it must not be broken at anytime.
'You must never date your friends ex. thats the cardinal rule of friendship'

'You broke the cardinal rule didn't you? Spies don't fall in love' - NBC's Chuck

'Never date a co-worker, its the cardinal rule. because when it all blows up in your face. your screwed'
by Keyls April 06, 2010
 
2.
In regards to friendship, the cardinal rule is never dating your friend's ex-boyfriend. When a close friend and their hunny break up, being a friend means you dont try to date, mack, get with, or hook up with the ex. Breaking this rule defiles friendship and ruins bonds and all friendly love with the rule breaker in the circle of friends.
After Max broke up with Sid, Alexis, a 'friend' of Sid, broke the cardinal rule and started dating Max less than a week after the break up. Sid and Alexis' friendship was ruined and Sid soon learned that no one gave two squirts of piss about her emotions, regardless or what was said.
by Sidoese February 04, 2009
 
3.
An unwritten rule, that is collectively known and abided by society as a whole.
"If one gest jumped by the Bra Boys in Maroubra's Last Goon Party, and you quickly realize that there is more than one guy, one can exclaim the cardinal rule '1-V-1', meaning the gang must give you a fighting chance, taking you on one at a time."
by bigberg June 09, 2014