Show One, Show All ( poker )

Most card rooms have a rule, generally referred to as "show one show all," that if a player shows their cards to anyone at the table they can be asked to show everyone else (even if they would ordinarily not be required to show their hand). This usually comes up at the end of a hand that did not reach showdown (e.g., if a player shows a friend a successful bluff). Obviously showing one's hand to someone else who has cards is illegal for more reasons.