They can’t take four
This Real Deal, modified slightly, is from the 2016 Spingold. South held:
♠ A J ♥ J 10 7 5 4 ♦ A 5 ♣ Q 9 5 2
At favorable vulnerability, his partner dealt and opened 1♦. South responded 1♥ and his partner rebid 2♦. Opposite a sound opening bidder, it would be OK to bid 3NT. In the Spingold match, both South players invited with 2NT. (Have you seen how light white-on-red opening bids can be these days?) At one table, opener passed, but at our table, he raised 2NT to 3NT.
You get the ♠3 lead (standard) and see:
With this lead, you have three sure spade tricks. Right-hand opponent plays the ♠8 and you win the jack.
As usual, I advocate counting winners at notrump. You have only six sure tricks (three spades, two diamonds and one club). If you take a club finesse and the suit behaves, that could get you to nine tricks (you’d need the ♣K onside and LHO to hold no more than three clubs).
Is there anything better?
Yes. The diamond suit. Only ♦Q–10–x–x-(x) offside would prevent you from taking five diamond tricks. That has got to be a better chance than the club suit, which is at best 50-50.
If diamonds produce five tricks, you have five diamonds, three spades and the ♣A. Yes, there is a slight danger of losing a diamond and four heart tricks, but more on that later.
At trick 2, you unblock the ♠A (you did, didn’t you?) and then play the ♦A. It goes low, low, 10. That’s pretty good news. You are now assured of at least five diamond tricks. On the next diamond, LHO plays low. Should you finesse?
Without getting into the psychology of the ♦10 – would RHO play it holding ♦10–x? – there is a clear reason to reject the finesse. It has to do with the heart suit.
With such a heart holding, you have to understand what layout would lead to you losing four heart tricks. The key is to realize that if LHO is on lead, you are completely safe. If LHO leads a high heart, all is well. If he leads a low heart to RHO, and a low heart comes back, you can split your ♥J–10 and LHO will be on lead. You will be protected.
What if RHO is on lead to start hearts? Now, you could have a problem. RHO leads a low heart forcing your 10 or jack. Then a low heart from LHO could go to RHO’s honor and another heart could come through you. If LHO has four hearts with two honors, you could have troubles.
All of that reasoning in the heart suit (which becomes routine with experience) dictates your play in the diamond suit. You don’t want RHO to win a diamond trick. If you finesse and lose to a doubleton queen (in this case ♦Q 10 doubleton), you could lose four heart tricks. Accordingly, you should go up with the ♦K; if RHO has ♦Q–10– x, you just have to hope for the best. East actually has ♦Q 10 doubleton. Look at what happens if you finesse:
After South won two spade tricks and played the ♦A, drawing East’s 10, he played a diamond to the king. The fall of the queen just meant an overtrick. Had declarer finessed, he could have been defeated. Notice that East has to shift to the expert ♥9 to avoid blocking the suit, and West would have to read the position, winning his queen and returning a low one to the king.