[gprentice]

On this hand should south cover with the queen when declarer leads the JS towards the closed hand.
The contract is 1H. After two rounds of diamonds, south leads JC won by north with KC, north leads back 7D won by east.
Declarer then leads JS. Should south cover with the queen?

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[awm]

No. You can see that dummy has the nine so covering will not help if declarer has the ten. I suppose it’s possible that declarer has no ten and is planning to let the jack ride, but this would be a very strange way to play the suit.

More likely is that declarer has KTx or ATx where he will not be able to return to dummy and repeat the finesse, or he has KT tight like here where covering removes a guess.

[gprentice]

Why would leading the jack be a strange way to play the suit? Declarer might have AKx and partner the ten, in which case you need to cover. Someone said to me that leading the Jack is like a Chinese finesse and that 95% of the time, declarer will have the ten because not many people try the Chinese finesse. But a Chinese finesse is when you want to trick them into not covering, but here you do want them to cover so you don't have to guess where the Ace Queen are. I can't see any reasoning that would help me not cover the jack with the queen other than guessing that declarer doesn't have both ace and king, in which case you can play low because declarer can't get back to the table.

[smerriman]

Perhaps it's true that if declarer holds AKx vs J9xx, the percentage play is leading the J. This only works if South holds the queen and North the ten, but a ~25% chance is a bit more likely than the queen dropping in 2.

But even if declarer always did that (and I doubt many would), there are twice as many hands where they hold ATx or KTx, when covering will cost you.