We will discuss two important notions: first, decoys and then 100% and 0% finesses. 

Let’s start with the decoys. These are honors that may lead you on a false track, while discovering the winning line of play would be easier if you had a small card instead!

South plays in 4 Spades and gets a Club lead. He wins and draws trumps in three rounds before eliminating the Club suit.  If he then falls victim to the charm of the Queen of Hearts and tries the Heart finesse, he’ll go down. West wins and plays another Heart, leaving declarer forced to open up the Diamond suit and to lose three tricks.

The line of play that is successful all the time (once you know trumps to be 3-2) consists in playing the Ace of Hearts followed by the Queen of Hearts. Now, whoever gets in with the King of Hearts (no matter if it is East or West) will have to either play a Diamond (for an automatic trick for declarer) or give you a ruff and discard. Making this contract with the Heart 2 instead of the Queen would be much more obvious!

Let's now look at 100% and O% finesses with the help of some exercises:

Lead: ♣Q. You take the Ace, draw trumps in two rounds, then cash the Club King followed by Ace, Queen and five of Diamonds.

What if West discards or follows small?

If he discards, it’s easy: take the King and continue with the 10, discarding a Spade.
East, in hand with the Jack of Diamonds, will have to play a Spade or give you a ruff and discard. If he plays a small card, insert the 10 to make sure you make against any layout:

• If the 10 holds the trick, that’s the twelfth.
• If East wins with the Jack, he’ll be on play and again has to either play a Spade or give you a ruff and discard. From the moment that the finesse against the Diamond Jack allows you to make even if it fails, we call it a "finesse at 100%"!

Lead: ♣A. 

East follows with the 7 and West insists with the Club King.

You ruff, draw West’s trump and ruff dummy’s last Club.

How do you continue?

If you take the Diamond finesse and it works, you make your contract, but if it fails you will inevitably go down because the Heart Ace cannot be onside anymore! West, a passed hand, would then show 9 or 10 HCP and have no more space for an Ace.

That’s why it’s much better to cash the Ace-King from the top. If West holds the Diamond Queen, you make if it falls in two rounds and if East has it (and the finesse would have been successful), you still make because the latter will be on play after the third round of Diamonds and then has to play a Heart or give you a ruff and discard.

This finesse, which has absolutely no added value, no matter if it works or not, is called a “finesse at 0%”.

This article was written by Marc Kerlero and was originally published in Bridgerama+.