Given the auction it should be clear that declarer holds a long diamond suit and the A.  If declarer also holds the A, then the contract cannot be beaten.  Therefore East should play his partner for the A and concentrate on knocking out the A.  With the play of the 9 to the first trick, you may be tempted to continue with the J, but this would be a fatal error if declarer started with A 9 X.  Declarer will run the J to the Q, thereby retaining the vital entry to cash the diamonds once the K has been eliminated.  So East must continue with the K, forcing out the A early. Now careful defence ensures that there is no genuine play for the contract. 

N.B.  Note that 3NT played by North cannot be defeated, even if East finds the inspired lead of the K.  Declarer can duck this and take the second round in hand with the Q, retaining the vital A as an entry to cashe the established diamonds.

*  North's 2NT was 'Barron', showing 15+ HCP and at least four-card support.  The 3 and 3 rebids were cue-bids showing first-round control. 

In a recent duplicate, N/S  reached the reasonable 6 slam and West led the K.  Declarer drew trumps in two rounds and cashed the top three s before exiting with a .  When West takes the 10 he cannot afford to exit with a or else he concedes a ruff and discard.  However a switch to a low will allow declarer to play for divided honours and finesse each defender in turn.  The only hope is to switch to the Q and hope that declarer plays you for both honours.  It is the old story - if you give an opponent the chance to go wrong, sometimes he'll take that chance.

This hand was played in a Teams event some time ago.  Both N/S pairs reached the reasonable contract of 4.  At the first table West led the 10 and East encouraged with the 8 when declarer covered with the K.  Declarer switched to the 10 and ran it when East played the 3.  West won with the Q and returned a second , taken by East with the A.  A third came back.  Declarer ruffed low and had to go one down when he was over-ruffed.  What a chance missed!  If declarer had simply discarded his losing rather than ruffing, he could not be defeated.

At the second table West recognised that getting a ruff  was unlikely to gain, since he (probably) had two trump tricks anyway.  Accordingly he looked to establish tricks elsewhere.  His opening lead was the 9 and East discouraged.  Declarer played a to the King and ran the 10 to West's Q.  West wasted no time in switching to a low , recognising that any tricks would have to be established quickly in case declarer held the A.  This would have been expensive if declarer held A Q and did not hold the A, but it is still much better defence than was found at the other table.  As the cards lie it was safe to exit with a passive , but this would also have cost if declarer held the A.  An equal score for a much better defence.  No justice!

On a club lead the answer is easy, since you can cash your two club tricks and wait for your two Aces.  Given the auction partner is far more likely to lead a diamond.  Clearly declarer is relatively weak in high-card strength and partner must have a near-yarborough, just possibly holding the Jack of clubs and/or the Jack of hearts, but nothing else.  Declarer is known to hold at least six spades, headed by the King and Queen.  For his bid he must also hold the two red Kings.

From the lead of the 10 of diamonds, South holds a singleton or doubleton (with three or more he would have led second-highest) so declarer holds K X X (X).  This places him with somewhere between two and four cards in hearts and clubs combined.  If he holds only two combined the contract cannot be defeated since he will have (at most) a spade, a heart and a club to lose.  To have any chance to defeat the contract declarer must have two club losers and South must hold the Jack of hearts.  If this analysis is correct then a passive defence will succeed.  North should simply return diamonds every time he is on lead, stripping declarer of his entries to dummy before he has a chance to set up a long heart for a club discard.  Note that if declarer draws the trumps, North should only go up with the Ace if the Jack is played from the dummy.  This line fails only if declarer has a singleton club or if he makes the inspired play of dropping the King of hearts under the Ace and later finesses South for the Jack.

But what if a heart is led?  Now a passive defence will not do, since declarer can simply force out the Ace of trumps before setting up the long heart for a club discard.  North must switch to active overdrive and return the Queen of clubs at trick two!  Now when in with the Ace of trumps North will underlead his Ace of clubs to his partner's Jack.  It should not be too difficult for South to lead a second round of hearts for the ruff.

Notice how damaging to the defence a passive lead of a trump would be.  Now the only defence is to shoot your partner and hire a good barrister, who should be able to get you off with community service!  Indeed, if the judge is a bridge player he may consider that shooting your partner already constitutes a service to the community and will let you off with a warning!

West has opened with a weak 2, showing a six-card suit and 6-10 HCP. North/South were playing Lebensohl style responses over the take-out double and therefore South's 3 response was forcing for one round and could have been only a four-card suit.  North had little choice but to rebid 3NT and hope for the best.

Knowing that West held six spades, declarer took the first trick and switched to a low diamond to the Queen.  Clearly the defence would only succeed if West got in to cash his spade suit.  So East ducked the first diamond and when declarer played a second diamond to the  Ace, East hoped his partner held the Jack and dropped his King under the Ace.  Now there was no play for the contract, since declarer could not establish a ninth trick without letting West in to run his spades.  Well defended.

The sharp-eyed among you will recognise that this is exactly the same hand as in 'Test Your Play (7)' and, in fact, the deal arose in a Teams event.  Needless to say, defeating 3NT when partners brought home a dodgy 4 led to a hefty swing.  It just goes to show that exponents of scientific bidding don't always land in the superior contract.

West leads the  Ace.  It is clear that three quick tricks are available to the defence unless declarer has a singleton club (two clubs and the  Ace).  Therefore the contract seems to depend on declarer's guess in the trumps suit.  Will he finesse the Jack of trumps or will he run it?

There is one additional chance, however.  If you deceive partner about the position in the club suit, you may induce declarer to go wrong in the trumps.  You should encourage partner to continue clubs by playing the 8 under the Ace, following with the 3 when partner cashes the King.  Partner will play for a doubleton and continue with a third round.  With luck and a fair following wind declarer will also play you for a doubleton and will elect to ruff higher than necessary in the attempt to locate the Queen of trumps.  Whether declarer ruffs with the King or the Jack (or 10) makes no difference, since he must have at least the King and Queen of diamonds and the King of hearts for his opening bid.  Therefore he has a certain diamond loser unless he has precisely a singleton  K and  K Q doubleton, in which case he will be able to park his two diamond losers on the  A Q.  If declarer does ruff the third club high he will have a certain trump loser.  Essentially what your naughty deceit does is prompt declarer to take an early wrong view in trumps.

It must certainly be correct to lead a trump since your King is adequately guarded.  North has shown a marked reluctance to support his partner in this auction.  He was not even prepared to bid game over South's reverse, despite responding initially at the 2-level, thus guaranteeing reasonable values.  The likelihood must be that he is short in both of his partner's suits, with a slight preference for spades rather than hearts.  If dummy comes down with two spades and a singleton heart, a trump lead will gain a heart trick provided a second round of trumps is led when in with the Ace of hearts.  Even if the dummy has three spades (unlikely because he surely would have raised directly to game over 3) a trump lead could still well be correct.  After all, East could hold the singleton Queen!  Assuming declarer captures the Queen with the Ace of trumps, you will be able to draw two more rounds of trumps when in with the Ace of hearts, thus setting up the second heart trick for the defence.

The 5 response to the Roman Key-Card Blackwood showed two controls and the Queen of trumps, assuming the agreed trumps to be diamonds. 

Clearly there is a strong possibility that East is void in diamonds and so a diamond lead might well earn an immediate ruff.  However, if diamonds are not led declarer might well need to take a first-round finesse of the Jack to bring the suit in for no losers.  If declarer can get the suit wrong he will, since he is extremely unlikely to play for a 5-0 break unless you lead the suit.

The second consideration is partner's holding.  Assuming that he is void in diamonds, it is likely that he has at least ten cards in the black suits.  If he has an honour in both suits he may well be subjected to a squeeze after a diamond lead, since he will have to find at least three discards on the diamonds and anywhere between three and five more discards on the trumps, depending on the length of declarer's trumps.  If the declarer has both the Ace and King of spades you are unlikely to beat the contract anyway, since either declarer or dummy or both hold no more than a doubleton spade.  So you decide to play your partner for the  King and lead a spade.  Which one?  The 10.  This will deter partner from playing the King if declarer does not cover with the Jack if the dummy holds J X X and declarer holds the singleton Ace.  This avoids you being squeezed in spades & diamonds on the play of the hearts and clubs.

A difficult problem, but well worth the effort to solve.