CARDS EXPOSED BEFORE PLAY.
65. After the deal and before the declaration has been finally determined, if any player lead or expose a card, his partner may not thereafter bid or double during that declaration,[14] and the card is subject to call.[15] When the partner of the offending player is the original leader, the declarer may also prohibit the initial lead of the suit of the exposed card.
66. After the final declaration has been accepted and before the lead, if the partner of the proper leader expose or lead a card, the declarer may treat it as exposed or may call a suit from the proper leader. A card exposed by the leader, after the final declaration and before the lead, is subject to call.
CARDS EXPOSED DURING PLAY.
67. After the original lead, all cards exposed by the declarer’s adversaries are liable to be called and must be left face upward on the table.
68. The following are exposed cards:
(1) Two or more cards played simultaneously;
(2) a card dropped face upward on the table, even though snatched up so quickly that it cannot be named;
(3) a card so held by a player that his partner sees any portion of its face;
(4) a card mentioned by either adversary as being held in his or his partner’s hand.