In trumps, with K Q x x; Dummy having A J x x; play the Queen. If Dummy wins with A, play a small card for the second round, and he may refuse to put on the J. The declarer not having the 10, would make Dummy cover; but nothing is lost if he does, and it marks the 10 with your partner.
With King and others of a suit in which Dummy has not the Ace; avoid leading the suit until the Ace has fallen.
With King alone, play it if Dummy has the Ace; keep it if he has not.
Trumps. If a player in this position is strong in trumps, he should keep quiet about it and let the maker of the trumps develop the suit. False-carding is perfectly legitimate in trumps, and will deceive the declarer more than your partner.
End Games. There are cases in which it is necessary to play as if partner was known to possess a certain card, for unless he has it the game is lost. For instance: You want one trick, and have Q 10 x x, Dummy having K x x, of an unplayed suit. The Queen is the best play; for if partner has any honour you must get a trick; otherwise it is impossible.
You have K x in one suit, a losing card in another, and a winning card. You want all four tricks to save the game. Play the King, and then the small card; for if your partner has not the Ace and another winning card you must lose the game.
You have a losing trump, and Q x x of a suit in which Dummy has K 10 x. If you want one trick, play the losing trump, counting on partner for an honour in the plain suit. If you must have two tricks, lead the Queen, trusting your partner to hold Ace.
Leading up to Dummy. The best thing for the third hand, or pone, to do, when he does not return his partner’s suit, and has no very strong suit of his own, is to lead up to Dummy’s weak suits, and to lead a card that Dummy cannot beat, if possible.
The general principle of leading up to weakness suggests that we should know what weakness is. Dummy may be considered weak in suits of which he holds three or four small cards, none higher than an 8; Ace and one or two small cards; or King and one or two small cards. In leading up to such suits, your object should be to give your partner a finesse, if possible; and in calculating the probabilities of success it must be remembered that there are only two unknown hands, so that it is an equal chance that he holds either of two unknown cards. It is 3 to 1 against his holding both, or against his holding neither. Of three unknown cards, it is 7 to 1 against his holding all three, or none of them; or about an equal chance that he holds two of the three; or one only.