First, suppose you lead to the ace, LHO follows small, and RHO plays the nine. Then you come back to hand and lead the six, LHO playing small again. The queen and ten are still out, and there are three options: play the king, succeeding against RHO's Q9; play the jack, succeeding against 109; or play the three, succeeding against singleton 9. Q9 or Q10 are two distributions, so are 9 or 10, 109 is one distribution. So apparently one shouldn't finesse the jack, but it's roughly a toss-up between playing for the drop and playing the deep finesse. (It's relevant that RHO can't afford to play the Q from Q9 even if he knows you need four tricks from the suit, because he doesn't know you've got the spots for the deep finesse.)
Second, on any other combination of cards (unless the queen has dropped or someone has discarded), the normal and sensible line is to finesse the jack on the second round (detailed analysis, of the sort I've laid out here before, shows that some unlikely defensive strategies can make a different approach better, but they can't make this approach worse).
However, if one chooses to play for the drop in the first case, there's a problem: LHO can deflect you from this line into a losing finesse by playing high on the second round with 1043 (or 943). So the right thing to do is to take the deep finesse.
This does have some practical relevance: almost the exact combination came up on board 118 of the 120-board US teams trials final. At the time Diamond was leading Nickell by 5 IMPs, and the last two boards were to be flat, so this hand decided the match.
- Three or more clubs
- Natural and forcing
- Minimum, three or four clubs
- RKCB for clubs
- One ace
- Queen of clubs?
- Precision: 11-15, no 5-card major
- Usually denies a 4-card major
- 11-13 balanced
- Intended as a slam invitation with 4-4 in the minors
- Pick a slam - Bathurst didn't know what 4 meant, but he was a bit better than minimum, so...
- Moss knows what he intended by this
In the open room, Bathurst and Moss had a murky auction to a poor 6NT, and Rodwell led the ten of hearts. Bathurst won with the ace and led a spade to the four, king and ace. Meckstroth returned the three of spades to the queen and five. Now declarer led the five of clubs to the two, ace and nine, came back to the queen of hearts, and led the six of clubs. Rodwell played the four, and Bathurst was looking at the situation as discussed above. (I'm sparing you my thoughts on the difference between AKJ2 and AKJ3.)
So, giving expert consideration to the arguments, he ran the six? No he did not, he played the jack, and with good reason. Suppose you run the six and it holds. Now you have eleven tricks, and the best chance for a twelfth seems to be the diamond finesse. But if you're going to take the diamond finesse anyway, putting the jack in may well not cost even when clubs are 4-1: If South has three diamonds you've got four tricks in the suit once the finesse succeeds, if he has four or more he'll get squeezed in the minors, and if he's got two North may get squeezed in the pointed suits.
North discarded a heart on the jack of clubs, so declarer cashed the king of hearts throwing his club, and the king of clubs, crossed to the king of diamonds, and cashed the jack of spades throwing dummy's club. Evidently there had been no squeeze, but the diamonds could still be 3-3. Declarer led a diamond towards dummy's AJ5, and North, holding a spade winner and Q8 of diamonds, claimed two off. That was 13 IMPs and the match to Nickell.
That's a reasonable line, but Bathurst could have made his contract by running the six of clubs, then squeezing North in spades and diamonds - worth considering once South shows four clubs and probably some heart length. South, on the other hand, could have prevented this by covering the six of clubs: it's unimaginable that he would do that since he can't know that declarer needs to do more than make four clubs tricks (QJ9x of spades would be enough). East, on the third hand, could have made by cashing a top club at trick two then playing the king of spades from dummy, saving an entry, before getting everything else right. And South, on the fourth hand, could have beaten to contract at trick one by leading a diamond to break up the squeeze. "It's poor that they're still playing single dummy in such a big event", as my correspondent remarked.