Reading partner's mind...

Both sides vulnerable at matchpoints, you are West.

♠ 4  ♥ J 3  ♦ A 10 9 8 6 4 3 2  ♣ 5 2 

South	West (you)	North	East
1 ♠ Pass	3 ♦ Pass	4 ♣ 4 ♠	4 ♦ All pass

You lead the ♦A, and dummy comes down with:

	North
	♠ A K Q 9 ♥ 6 5 ♦ 7   ♣ K 10 9 8 6 3
West (you)
♠ 4  ♥ J 3 ♦ A 10 9 8 6 4 3 2 ♣ 5 2

You lead the ♦A, singleton ♦7 from dummy, partner plays the ♦K, and declarer plays the ♦J. What next? 

ANSWER:  Play the ♥J.  Partner is screaming for a heart switch.

THE PRINCIPLE: With the singleton diamond in dummy, continuing diamonds looks like a waste. When it’s clear that continuing the suit is a poor choice, partner’s card is a suit preference signal. A high card indicates a preference for the higher-ranking of the other two non-trump suits; in this case, a preference for hearts rather than clubs. So, you lead a heart as requested; from jack-small, the jack is the better choice.

The complete hand:

North ♠ A K Q 9 ♥ 6 5 ♦ 7   ♣ K 10 9 8 6 3 West (you) East ♠ 4  ♥ J 3 ♦ A 10 9 8 6 4 3 2 ♣ 5 2 ♠ 8 3 2 ♥ A 10 9 7 4 ♦ K Q ♣ J 7 4 South ♠ J 10 7 6 5 ♥ K Q 8 2 ♦ J 5 ♣ A Q

It’s now or never for partner’s ♥A. If you don’t lead a heart, declarer will win any other return, draw two rounds of trumps, play the ♣A and ♣Q, and cross to dummy with a spade. Then she’ll cash four more club tricks, discarding all of her hearts. Making six spades will be a good matchpoint score for the opponents, but not so good for you.

If you can’t read partner’s mind, read her signal.

BUT WHAT IF PARTNER’S SIGNAL CALLS FOR THE WRONG THING TO DO? Do it anyway. Really. Unless you’re 200% positive that partner is wrong, do it anyway. The benefits to partnership harmony, knowing that you’re both paying attention and trusting each other’s signals, far outweigh any short-term gain of maybe being right this time.