Source: The Free Lance-Star – Aug 7, 1992
You are declarer with the West hand in six hearts and North leads the jack of clubs. You play the queen from dummy and win South’s king with the ace. When you cash the A, king of hearts, North, turns up with the Q-10-8, so you must lose a trump trick. How would you proceed from here?
Obviously you must try to dispose of both of your club losers before North can gain the lead with a trump to cash the setting trick in clubs. If all goes well you can deposit one club on a spade and another on dummy’s fourth diamond. The only problem is which suit to play first.
Strangely enough, the proper method of play calls for tackling the diamonds first. There is an excellent reason for this, as can be demonstrated by assuming that the North hand looks something like this:
If you started by playing the K-Q-A of spades, North would ruff the third one and cash the ten of clubs to put you down one. But if you started by playing the A-K-Q of diamonds and learned that North originally had four diamonds (after South showed out on the third round) you safely continue with the fourth round of diamonds and discard one of your club. You would then play the K-Q-A of spades and just get under the wire by discarding your  remaining club loser on the third round of spades. Your only loser on this line of play would be a trump.
However, if North turned up with precisely three diamonds, you could not afford to continue with a fourth round of diamonds since North could ruff and cash a club to defeat you. In that case you would next play three rounds of spades hoping for the best, and then lead dummy’s last diamond to get rid of your last club.
The underlying principle is simple enough. It is impossible to make the contract if North has less than three diamonds, so you proceed on the assumption that he has three or more.You therefore lead diamonds first to see exactly how many he started with, and adapt your play according to what develops. Leading diamonds first  can never be the cause of losing the contract, but leading spades first sometimes will be.
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