When you return your partner's lead, the card you should choose to lead the second round depends on the number of cards of the suit you have remaining. Thus, if you remain with three cards, you must have had four at first. You therefore had strength in the suit, and you should return the smallest of the three remaining cards, agreeably to the principle that with strength it is to your advantage to retain the command in your own hand. If you remain with two cards only, you should return the higher one, to strengthen your partner; and, similarly, if you have discarded one of a four-suit, and are left with two onlyat the time you return it, you have destroyed the numerical power of your suit, and should therefore treat it as a weak suit, and return the higher card of the two remaining in your hand.
The advantages of this principle are numerous. In the case that you and your partner are both numerically strong, the return of the lowest prevents himfrom finessing in a suit which must be trumped third round. Further, if your hand is weak, you naturally return a suit in which you infer that your partner is strong. You then return a strengthening card to get a high card of your partner's strong suit out of his way, and you enable him to finesse if he thinks proper, and so to keep the command of his suit in his own hand.
It is true that with two small cards only (say the five and the six) you do not strengthen your partner by returning the six. But there is a collateral advantage in keeping to the rule even with small cards—you enable a good partner to calculate how many you have left of the suit, and often where the remainder of it lies. Thus, your partner leads a small card of a suit of which you have king, three, and two. You, as third player, put on the king. If you return the suit, you return the three, and not the two, when it ought to be inferred, either that you have returned the smallest of a suit of four or more, or that you have no more of the suit left, or the two only. When your two comes down in the third round it ought to be certain that you have no more. If your partner has confidence in you, he can often calculate what you have left before the third round is played; thus, in the above instance, your partner, not having the two himself, and seeing that it does not drop from the adversaries, concludes, with tolerable certainty, that you remain, after the second round, with the two and no more.
There are two exceptions to the rule of play above stated: 1. When you hold the winning card youreturn it, whatever number of cards you hold, lest it should be trumped the third round, or, your partner, imagining it to be against him, should finesse; and 2. When you hold the second and third best, in plain suits, you return the highest. Thus, suppose you have queen, knave, ten, and one small one of a suit of which your partner leads a small one, you (third hand) put on the ten, which is won by (say) the ace. If you afterwards return the suit, you should return the queen, for you not only force out the king, if against you, but you also do not block your partner's suit, should he have led from great numerical strength, say five cards to the nine, an advantage which you lose by returning the small one.
It should also be observed that, occasionally, when you return your adversary's strong lead, you do not lead the higher of two remaining cards, especially if you hold the second best guarded. For example, you are A; Y is your left-hand adversary. Y has led a king, which was won by the ace, leaving Y with the queen and others. You remain with knave and one small one. If you are driven to return this suit, you should return the small one. The queen will probably be put on second hand, and you will remain with the best.