There's an old bridge maxim which goes like this: "Cover an Honour with an Honour". Here's where this might apply. When declarer plays an honour from dummy, and you, the first defender to play to the trick, hold a higher honour, it might be an advantage for your side to cover with your higher honour. e.g. You hold: (see below)
Declarer plays the ♦J from their hand, you hold the ♦K5 and dummy has the ♦AQ764, Do you cover the ♦J with your ♦K?
Answer: Yes, you hope it will promote a winner in partner's hand. Declarer probably has a doubleton diamond, and your partner may therefore hold four diamonds. This could be the layout: Dummy
If you cover the ♦J with your ♦K, you will promote two diamond winners in partner's hand (the ♦10 and ♦9).
When declarer plays an honour like this, they usually WANT you to cover, so look at dummy to see if the 10 and 9 are there. Don't cover if you can see those cards in dummy, because you can't be promoting tricks for your side by covering.
When declarer leads the ♦J you might as well play the ♦5, hoping that declarer will change their mind and play the ♦A!
Another time to be careful of covering an honour with an honour is when you see two touching honours in dummy, eg
if declarer calls for the ♦Q from dummy, don't cover the first honour with your ♦K. Wait until declarer calls for the second honour, the ♦J.
Why don't you cover the first honour? Because you might make NO tricks in this suit, if partner is holding the ♦10. The play will go: ♦Q, ♦K, ♦A, ♦4.
Now ♦2, ♦5, ♦9, ♦3. Next for the third trick, declarer will win their ♦J. (Not what you hoped for by covering the first card).
But if you waited to cover the second trick, when declarer has "run" the ♦Q, and it holds the trick, when you cover the ♦J, partner's ♦10 will win the third trick.
It's very hard to know exactly when to cover an honour, but how many cards declarer and dummy hold in the suit will also give you a guideline. Let's say you knew they had nine cards between them, and declarer played the ♦J
If you cover this with your ♦Q, you will drop partner's singleton ♦K.
So, while we won't succeed every time, if you know declarer has length in the suit, be less inclined to cover the honour with an honour, and ditto if you are able to see the middle cards sitting in dummy. (Declarer is playing the honour because they WANT you to cover!)
Otherwise, you should cover honours with honours, in the hope of promoting winners for your side.
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