Source: Negative Inferences by Marshall Miles
Perhaps you won’t consider the following hand a problem. True, it shouldn’t have been. But it is the sort of hand that you get frequently, so it is important to understand the general principles involved.
South opens a strong NTfollowed by three passes. You lead the deuce of diamonds to partner’s queen and declarer’s king. Declarer leads a low spade and you play small letting partner win the king. He returns the ten of diamonds, covered by declarer’s jack and your ace. Yo cash the 9 of diamonds and partner discards the nine of spades.
What do you play next?
Surely not a spade. Nor a diamond to the eight since the dummy problably has no entries. You wish that partner was on lead. Why didn’t he return a club or a heart?
He didn’t know which to return since he probably has roughly equal holdings in both suits. How do you know this?
Because if he had good hearts and nothing in clubs, he would have discarded a discouraging club. If he had good clubs and nothing in hearts, he would have discarded a discouraging heart. His failure to discard either a club or a heart means that he has honors in both suits. (Also his discard of his higher remaining spade should be a suit-preference play.)
Anyway, you are going to have to lead a heart or a club sooner or later, so you might as well lead one now. I would choose a heart for two reasons. Your hearts are better than your clubs, and partner’s spade discard is probably a suit-preference play. Which heart?
Partner ought to have about four points, based on the bidding. If he had the ace of hearts, he would hold nothing in clubs and would probably discard a discouraging club. So you might as well lead the king (or jack) of hearts.
Partner has 
Q 10 x x and 
Q 9 x x.
Eventually, you or partner will have to lead a club, enabling declarer to win his fifth trick. At the table, West led the fourth round of diamonds, letting dummy’s eight win. This was a bad play which was compounded later. East discarded a club on the fourth round of diamonds, and when West won a spade trick he led a club rather than a heart. At notrump, one holds his probable winners and discards his losers, so, West should at least have returned a heart when he got on lead.