There is an old adage: “Cover an honor with an honor.” Note that this applies only to a
defender and is not always correct.

Here are some guidelines for what to do when dummy is on your right and you are in second seat (the same concept applies regardless of whether this is a notrump or a suit contract):

Dummy (North)
Q52 You (East)

Declarer leads the queen from dummy and you have the K109. No declarer worth his salt would lead the queen without also owning the jack. Thus declarer must have the AJx. What should you do? You must cover with the king, so that your ten will ultimately become a winner.

Dummy (North)
Q52 You (East)

The queen is led from dummy. This time you do not own the nine or the ten, so you have to hope that your partner holds a worthwhile card in that suit.
On this layout, you must cover with the king, so that West’s ten can develop into a winner. If you do not cover, declarer will repeat the finesse and score four tricks in the suit.
Thus, when dummy is on your right, it is most often correct to cover an honor with an honor.

Let’s see the entire suit laid out:

Dummy (North)
Q52 You (East)
1083 K64

Now let’s look at a different scenario:

Dummy (North)
QJ109 You (East)
873 K542

The queen is led from dummy. When dummy is on your right and has more than one touching honor, you must cover the last honor. (Here, the ten and the nine have grown up into being classified as honors, since they lie in sequence with the queen and the jack.) If you cover the first honor led, then declarer has four sure tricks. If you do not cover, then declarer may get only two tricks in this suit.

Let’s have a look at a variation in the context of a full deal:

4 3 2
K J 5
Q J 9 3
A 4 3
8 6 4
10 8 6
J 10 9
9 8
9 7 3 2
K 7 2
8 7 6 5
10 7 6 5
A Q 10
A 5 4
K Q 2

South 1NT—-North 3NT —-All Pass

Opening lead: Ace Spades
West, your partner, takes the first four spade tricks and shifts to the Club Jack. Meanwhile,
declarer counts his sure tricks: three hearts, one diamond and three clubs. Not enough! He
realizes that his only hope is to score two extra tricks in diamonds. He wins with the Club Ace in dummy and leads the queen of diamonds. If you, as East, cover with the king, declarer will win with the D Ace and finesse your partner for the D 10, resulting in nine tricks. If you refuse to cover the D Queen, however, declarer will win that trick, but cannot now generate the three tricks he needs in the diamond suit. Your long-range plan should be to cover the second honor.

This will beat the contract.