This hand is taken from a famous book Spotlight on card Play by Robert Darvas and Paul Lukacs…

You are South, and your contract is 3NT. West leads the 6. Is the contract in sight? Stop and think. . Yes. You have two tricks in hearts, three en diamonds and four in clubs, totalling nine. In what order do you intend to bring home your tricks?

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It looks a good idea to play a low in dummy so as to take the trick cheaply in South’s hand as the Eleven Rule tells you East has no heart higher than the six. That would be all right if you could be certain that West has led his fourth highest heart. But perhaps he hasn’t; and then the Rule of Eleven simply won’t work, and you may well find your K forced out by the knave or queen appearing from East’s hand. You next cash South’s three high clubs, enter dummy with a diamond -for, of course, opponents hold up the ace- and cash the A.

Again you play diamonds, and the ace appears only on the third round of the suit. Now there is no entry in South’s hand for making the third diamond trick. You can no longer get nine tricks. Con you spot the mistake?

The first trick was taken in the wrong hand. You must resist the temptation to assume the West’s 6 is the orthodox fourth-high lead. ln this hand you dare not bank on the Rule of Eleven, and risk having to play your K prematurely. So you take the first trick with dummy’s A and thus ensure the safety of your K as entry to your hand for the third essential diamond trick after you have cashed your four club tricks in the way already described. That makes your contract nearly a certainty.