CARD COMBO: KQ10x opposite xxx
Yesterday, an opponent misplayed this combination: KQ10x opposite xxx.
Proper play is low to the King FIRST (unless you KNOW from the bidding that the Ace is behind the King—the only time when low to 10 on first round is reasonable). If the King loses, you play low to the 10 (finessing for the Jack) on the second round of the suit.
The challenge is when the King wins. The textbooks say to play low to the 10 next. However, knowing your opponents will help. If you KNOW (or strongly suspect) that the person behind the KQ10x would never duck the Ace, play low to the Queen on the second round. In a suit contract, it is unlikely that the person with the Ace will duck the second time when you play low toward KQ10x—particularly if you keep one low spot card hidden to try to mess up the opponents' count of the suit, so the 10 is most likely correct. In no trump, a good opponent will duck TWICE to give you a guess on the second round, but the textbooks say go with the ten.
Similarly, if a GOOD opponent takes the Ace on the first round of the suit in no trump, play that opponent for AJ doubleton. Otherwise, that opponent should have ducked smoothly on the first round to try to give you a guess. [It is also possible that opponent has AJx behind KQ10x.]
A related note: in a no trump contract, you have AJx behind KQxxxx in a Dummy which has only ONE outside entry (Ace in another suit), If Declarer plays low to King on first round of KQxxxx suit, DUCK—thus killing the suit if Declarer has only two cards in it. [Take the second round.] A skilled Declarer will duck the first round COMPLETELY to force out ONE of your stoppers; then play to King or Queen (after regaining the lead) to force out your second stopper before the ONE outside entry in Dummy is dislodged.