When you do not head a trick, you throw away your lowest card to economise your strength. Thus, with queen and two small ones, you would not throw the queen to king led. It is an error to suppose that it is of no consequence which card you play when you hold only small cards or cards in sequence. It is not of much consequence as regards merely the chance of making tricks; but it is of great importance in affording information to partner.
Thus, suppose the players to be as before, A, Y, B, Z. A leads the three of a suit, Y plays the five, B the four. It ought to be certain that B has no more of the suit, it being presumed that he, not being able to head the trick, throws away his smallest. If he afterwards plays the two, and it turns out that he previously played the fourthrough carelessness, his partner loses confidence, and gives up all hopes of drawing correct inferences from his play.
The principle applies equally to cards in sequence. Thus, say queen is led, and you (second hand) hold ace and king; if you put on the king, your partner gains the very important information that you have the ace also. For queen is not led from ace, queen, &c., so the leader cannot have the ace; the third hand cannot have it, or he would win the king; and the fourth, not having it himself, infers that you have it. If you put on the ace, not only could he not tell that you hold the king, but would assume that it lay with the leader's partner. The principle, though stated for the sake of convenience in respect of the second hand, applies to the third and fourth hands also. (For a fuller examination of this pointseeSection 12.)