Source: Ocala Star-Banner – 18 Jun 2003
John Boyle O’Reilly, an Irish author and poet, wrote this stanza:
“You gave me the key of your heart, my love: ‘ Then why did you make me knock? /
Oh that was yesterday, saints above! And last night I changed the lock!”
At the bridge table, finding the right key to open the door to the other hand may be vital. Here is an example. 
Sitting South. How would you plan the play in five diamonds? West leads a club. East winning with the queen and continuing with the king.
The declarer was Jan Wohlin (woe•lean) who died in 1999. He was on the Swedish team that won the 1952 European Championship, but is best-known for composing wonderful declarer-play problems. Wohlin could see 11 tricks via one spade, live hearts and five diamonds. But to cash those five hearts, he needed a dummy entry. And so anticipating the likely 3-1 diamond break, declarer discarded a low spade from the dummy instead of ruffing the second club.
Then, when East continued with the club ace, Wohlin ruffed in hand with the diamond nine. Now it was easy to draw trumps in three rounds, unblock the ace-king of hearts, cross to dummy by leading the carefully preserved diamond five to dummy’s six, and run the rest of the hearts.
Note that East defended well by playing the second club. If South had ruffed it in the dummy, the contract would no longer have been makable.