You are East defending 3NT, and your partner’s opening lead is the 10 of diamonds. You see this dummy and recollect the bidding shown:
Which card to you play and what is your plan? (See below)
Things are looking good. You have a sure entry with the ace of spades, and partner must have about seven HCP (why?). Since partner is surely leading from a perfect or almost perfect sequence, you should unblock with the jack.
Assuming declarer wins the 1st trick with the king or queen, which diamond will you play later when you are in with the ace of spades? This is something that comes up a lot, and many players get it wrong. East should play the four of diamonds, giving declarer remaining count. High-low shows an even number of diamonds remaining. Partner will know how many diamonds declarer started with and whether she can cash her remaining diamonds.