[twcho]

Suppose you have QJ63 and A984 in dummy and hand. No problem in entries to both hands. How should you play to give the best chance of bringing in the suit without loser? Suppose: (a) RHO cover the first trick with K; (b) RHO doesn't cover.

[jillybean]

Have a look here, it happens to be the first of many combinations:

http://forums.bridge...t=ST&f=5&t=5151

[MickyB]

If the Q holds the trick, there are six relevant holdings for RHO -

K7
K5
K2
K75
K72
K52

So, Kx and Kxx will occur with the same probability. As there is a non-zero chance that the defender would have covered with Kx, it's better to play for him to have started with Kxx and play the jack on the second round.

If RHO covers, there are four relevant holdings for him -

KT
K7
K5
K2

As we have seen above, there is no need for him to cover with Kx, so if he does cover you should, in theory, play him for KT; However, Kx is three times as likely as KT, so if you believe that the defender would cover more than one-third of the time when holding Kx, you should play him for that instead.

[han]

I couldn't find it Kathryn.

[twcho]

Thanks MickyB of his detailed analysis. That is what I have learned from his post (maybe wrongly interpreted):
1. You should not cover with Kx since a capable declarer will try to pinch down partner's 10x in the suit on 2nd round.
2. You should cover with Kx because he will try to pinch your presumed K10 doubleton if you covered with K (because declarer assumed that you will not cover with Kx).

So what is the proper action a defender should do in 2nd seat with Kx on hand? And what conclustion declarer can draw from the cover/non-cover from the defender?

[MickyB]

The best theoretical strategy for the defender is to cover *at most* one-third of the time. If he does this, declarer will pick up Kxx and KT, but never Kx. If he covers more than one-third of the time, it becomes profitable for declarer to start playing him for Kx instead of KT.

In practice, the best thing to do is to not cover with Kx, because if declarer believes you would frequently cover he will now pick up the suit. The flip-side of this is that if you have KT, you hope that declarer thinks you would frequently cover from Kx.

[twcho]

Not covering Kx runs additional risk of declaring picking up the suit without loser.

1. If declarer holds A9xx, once you cover, he has no chance to make. If you refuse to cover. He still has losing option.
2. If declarer holds A8xx, unless LHO cooperates by falsecarding in following 10 or 9 in the first round. Declarer has only winning option left.
3. If declarer only has A7xx, he will be forced to pick up the suit. of course, if this is the case, you will only have one possibility of Kx