Source: Charles Goren
1)
Diagram A
 A 6 5 4 |
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| Q 10 3 2 |
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With Diagram A you are trying to win three tricks. If you think that East has the king, you should play as in the previous example:
| A 6 5 4 |
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| Q 7 3 2 |
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With Diagram A your object is to win three tricks in the suit. This may be done in two ways, depending on who you believe has the K. If you think East has it, you should play the A and then lead a small one toward the Q. If you think that West has it, then your only hope is that it will fall in the second round of the suit. In that case you should play the A and a small one and duck the second round. The holding will then have been as in Diagram B following.
Diagram B
| K 6 4 2 |
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| J 10 8 |
A 9 |
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| Q 7 5 3 |
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(Watch Diagram A again) But if you think that West has the king, then it may be that East has the Jack, in which case you should lead low from the North hand and finesse the 10 after the play of the A, hoping that this will drive out the king.
2)
Diagram B
| K 6 5 4 |
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| Q 10 2 |
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The normal play with Diagram B, assuming plenty of entries, is the finesse of the 10 by declarer, hoping that East has the Jack. Suppose, however, from the bidding you are morally certain that East holds the Ace but are not quite sure as to the location of the Jack. A somewhat abnormal play will frequently be effective if you are able to determine the exact number of cards in the suit held by the adversaries. Suppose, for example, during the play of the hand you are able to determine that West holds four cards of this suit. That would leave East with only two cards, one of which you are morally certain is the Ace. The proper play is to lead the 4 from dummy and win with the Q. At the next trick the 2 is led and ducked in dummy, hoping the A will fall. The complete holding will be as follows:
| K 6 5 4 |
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| J 9 8 3 |
A 7 |
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| Q 10 2 |
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3)
| A 6 5 4 |
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| Q 10 9 8 |
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Your object is to take three tricks, but this combination differs slightly from the preceding one. If you can guess where the cards are you can make the proper finesse, but if you guess wrong you will lose two tricks. In order to avoid guessing in the long run it is better to play upon the theory that two high cards adversely held will usually be found in different hands. You should therefore lead first from the South hand, intending to lose the finesse to East if West does not cover and subsequently enter the South hand and finesse again. It does no matter who has the king or the Jack. Just so long as they are in different hands, you will succeed.
Diagram A
| A 6 5 4 |
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| J 2 |
K 7 3 |
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| Q 10 9 8 |
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Diagram B
| A 6 5 4 |
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| K 2 |
J 7 3 |
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| Q 10 9 8 |
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In either of these cases, if the A is played first and then a small one when East plays low declarer must guess whether to play the 10 or the Queen, and the wrong guess will be fatal. Where two finesse are taken, only one trick will be lost in both cases.