THE NO-TRUMP RULE THAT USUALLY WORKS
Jean Kerr, a humorist, author and playwright, said, "I think success has no rules, but you can learn a great deal from failure."
In bridge, there are some rules that will lead to success -- or, in contrast, if they are ignored, you will learn from your failure. But the game remains popular because there are so many deals on which one can calculate that the usual rule does not work.
What is the key rule for South in today's deal? He is in three no-trump, and West leads the spade six. (As a side issue, looking at all 52 cards, how must declarer play if West leads the heart jack?)
If South had bid two notrump over his partner's takeout double, it would have shown some 10 or 11 points. North, playing partner for six or seven points, cue-bid three spades, asking South to bid three notrump with spades stopped.
After a spade lead, declarer sees five top tricks: two spades and three diamonds. He needs to establish three tricks in clubs and one in hearts. But that means losing the lead twice, presumably once to West and once to East.
With two stoppers in their suit and two high cards to dislodge, duck the first trick.
South takes the second spade and plays a club. West can win, but doesn't have another spade to lead. (Note that if declarer plays a heart at trick three, West should put up his jack. Then East should take dummy's queen with his ace and return that suit, not a spade.)
On the heart-jack lead, South must play low from the dummy to get home.