North-South vulnerable.  
West deals.
                              North
                              S: A K 4            
                              H: K 7 2       
                              D: A 9 8 2     
                              C: K 5 3   
            West                             East
            S: J 8 7 5 3                     S: Q 9 2        
            H: 6 4 3                         H: A Q J 9 8 5
            D: Void                          D: 7 5 4             
            C: J 9 7 6 4                     C: A           
                              South                         
                              S: 10 6 
                              H: 10         
                              D: K Q J 10 6 3
                              C: Q 10 8 2 
          The Bidding:  
                   West   North     East   South                  
                   Pass    1NT      Dbl     Pass
                    2C     Pass     2H      3D  
                   Pass    4D       Pass    5D  
                   Pass    Pass     Pass        
          Opening lead: H3
     When the dummy was placed on the table, declarer became
aware of the fact that three no trump would have been a good
contract.  The likely result would be plus 600 so making five
diamonds would yield a similar score.  The problem would be in
making eleven tricks.
     East wins the first trick with the jack of hearts and
switches to a diamond.  Declarer plays the ten and West shows
out.  A spade to the ace allows declarer to trump a heart as he
prepares to both strip out the side suits and get a count on the
hand.  A small diamond to the nine is followed by the king of
hearts.  East covers and declarer trumps high.
     A spade to the king and a third spade from dummy, trumped in
hand brings declarer to this position:
                              North
                              S: Void             
                              H: Void        
                              D: A 8       
                              C: K 5 3   
            West                             East
            S: J 8                           S: Void         
            H: Void                          H: Q 9 8  
            D: Void                          D: 7                 
            C: J 9 7                         C: A           
                              South                         
                              S: Void 
                              H: Void       
                              D: 3
                              C: Q 10 8 2
     West has followed to three rounds of hearts so East cannot
have more than six hearts.  West has shown a void in diamonds, so
East started with three.  Both East and West have shown three
spades.  This leaves room for only one club in the East hand.  He
may have a fourth spade which would leave him without a club, but
there is nothing that can be done about that.  
     If East has only one club, it is important to see which club
it is before any important moves are made in that suit.  With
that in mind, declarer leads his last diamond to the dummy,
extracting East's last trump.  The three of clubs is called for. 
East plays his singleton ace of clubs and declarer can claim the
last three tricks.  Had the singleton been the jack, declarer
would cover and still be able to claim the last three tricks.
     Had declarer not counted the hand out, he may very well have
played a club from his hand to the king on the table, losing to
the ace and finding that he still had to lose the jack of clubs
to West.  By counting the cards he was able to learn that West
could have no more than one club and be able to make the right
play for the contract.
     This type of counting is just plain old fashioned hard work
that must be done on each hand as both declarer or defender.  The
only time you get to take a break is when you are the dummy. 
When you find yourself counting all of the hands, the part
scores, the games, and the slams, you will know that your game
has improved.
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