Some deals require a very careful touch for the best result to be achieved. South learned this the hard way when he went down in a four-spade contract he could have made.
West led the nine of hearts. East cashed the A-K and continued with the jack, declarer ruffing with the nine as West discarded a diamond. South then led a low trump to the queen, won by East with the ace. East continued with the ten of hearts.
By this time, declarer had the K-J-10-7 of trumps and West the 8-6-5. It did not matter whether South ruffed the heart high or low; sooner or later, West's eight would become the setting trick.
South would have made the contract had he played the hand more cautiously. After ruffing with the nine at trick three, he should have crossed to dummy with a club and returned a low trump toward his hand.
East would play the ace, and South the deuce. But when East then led the fourth round of hearts, declarer would be in a much better position to handle the situation. He would have the K-J-10-7 of trumps, just as before, but the outcome would be altogether different because dummy would still have the queen. He could now ruff with the 10 without establishing West's eight as a trick.
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A low trump to dummy's queen, followed by a diamond to the ace, would allow declarer to draw West's remaining two trumps with the K-J and so eliminate West's potential trump trick.
It is true that declarer was unlucky to run into a 4-1 trump division and go down one, but he should have protected himself against this possibility by making the first trump lead a low one from dummy.

