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You hold :

Scoring: IMP

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You open 1♠, 2♦ by West, Partner makes a negative double. East passes, You bid 2♥, West passes, Partner bids 4♥, all Pass.

The opening lead is A♦, and dummy hits with A765♥, K74♠, 1076♦,AJ4♣.

West continues with the K♦ and a third♦, ruffed by East with the J♥. Do you overruff or not? And how do you play after that?

[FrancesHinden]

We need to assume the spade finesse is right.
We have no losers to discard on this third diamond, so it's just a matter of what's
going on in trumps.

Let's think through the possible trump holdings.

If East has ruffed in from length, we do better to over-ruff, play a heart to the ace and a heart towards hand. That picks up all 3- and 4- card heart suits on our right. If we discard, we are then left guessing whether RHO started with 3 hearts (play them from the top) or 4 hearts (ace of hearts, heart finesse).

If East has ruffed from shortage, I think we are stuck unless it's QJ doubleton or J9 doubleton. If QJ doubleton, it doesn't matter. If J9 doubleton we need to over-ruff and run the ten of hearts, but this is a very obscure layout.

So, as RHO is likely to have longer hearts anyway, it looks fairly clear to over-ruff.

[FrancesHinden]

If LHO has singleton queen of spades and the queen of clubs we also gain by discarding on this trick.
But that also seems much less likely than the risk of mis-guessing whether trumps are 4-1 or 3-2.