The following are exposed cards:
1st. Two or more cards played at once.
2d. Any card dropped with its face upward, or in any way exposed on or above the table, even though snatched up so quickly that no one can name it.
3d. Every card so held by a player that his partner can see any portion of its face.
A card dropped on the floor or elsewhere below the table is not an exposed card.
If two or more cards be played at once, by either of the dealer's adversaries, the dealer shall have the right to call which one he pleases to the current trick, and the other card or cards shall remain face upward on the table and may be called at any time.