With A x x, Dummy leading Jack, play the Ace.
With any fourchette, cover the card led.
If Dummy remains with one or two small cards of a suit that has been led, and you have the best, play it on the second round. Dummy’s play is evidently for the ruff, and if the declarer has not the second best, your partner has.
If you have King, and only one or two small cards, Dummy leading Queen from Q 10 x x, play your King. You cannot save yourself; but you may make the 9 good in partner’s hand. If you have three or more small cards, do not play the King, for either partner or the declarer must be short in the suit. So if Dummy leads Jack from J 10 and others, play the King with a short suit. If partner has Queen you establish it; if not, you cannot make a trick in the suit.
With short suits it is usually best to cover an honour with an honour; but with several small cards, such as K x x x, Dummy leading a singleton Queen, you should pass.
With K 10 x, Dummy having J and others, play honour on honour; small card on small card, whichever Dummy leads.
It is often important for the second hand to cover with what is called an imperfect fourchette. A true fourchette is the card immediately above and below the one led; such as K J over the Q, or Q 10 over the J. An imperfect fourchette is the card above the one led, and another next but one below it; such as K 10 over a Q led, or Q 9 over a J led. Covering forces the opponents to play two honours to win one trick, and will often make an intermediate card good in your partner’s hand.
THIRD HAND PLAY. In addition to the methods of echoing on the partner’s leads of high cards in the suit first opened, third hand must be ready to adapt himself and his play to any change of suit and will require constant practice in putting himself in his partner’s place, asking himself what the object is in leading certain cards through Dummy’s hand. The inferences from the conventional leads should be sufficiently familiar to need no further explanation; but even good players occasionally overlook indications that partner holds certain cards. For instance: A leads a small card; Y, Dummy, holds Q x x, and plays Q. You play the King and win the trick. This marks not only the Ace, but the Jack in partner’s hand; because the declarer would not play a twice guarded Queen from Dummy’s hand if he had the Jack guarded himself.
False cards should be avoided by the third hand as much as possible. The declarer will give your partner enough to puzzle over without your adding to the confusion. There are some exceptions in trumps. For instance: You have K Q x; Dummy has A J x x, and your partner leads. Unless Dummy plays Ace, you should put on the King, and change the suit.