Defence play. Counting the enemy cards and points
To be a good bridge player, you must start counting points in opponents hands during the play of the hand. Did an opponent have a chance to open the bidding and they did not? If so, then they have less than opening count.
Did they overcall? This too takes a minimum amount of points; usually opponents will not overcall with less than about 8 points.
Counting Declarer's points and lengths is key to good defence. To this end, once dummy becomes visible, STOP and count how many points partner has. Subtract your HCPs, Dummy's and the points that Declarer has shown from 40. This will give you a good idea of how much partner can contribute to your joint effort.
Dummy: 
Qxx 
K10x 
Kxxx 
Jxx
You: KJx Qxx xxx Q10xx
After 1NT:2NT:3NT you ask and determine that 1NT showed 12-14 points. To give you the best chance, assume 13 HCPs in the Declaring hand. By subtracting 13, 9 (Dummy's HCPs) and 8 (your HCPs) from 40 we can infer that partner has ten HCPs. If in doubt regarding inferences about Declarer's HCP total, apply this general rule: Be optimistic at Rubber bridge, assume that Declarer is stretching to bid a close contract and accord Declarer the least number of HCPs that would be consistent with their bidding.
More inferences, did the opening leader lead a card, to his partner's ace, and trump the return of that suit? If so, he has 12 cards in the other three suits, and if you and dummy have only 6 cards in the lead suit, then opener's partner has only 7 other cards. If you have to put cards into one hand or the other, in order to know which way to finesse, put them in the long suits hand, the opening leaders hand.
At this point you might read through the following percentage table: It is not important that you memorize this table. It is, however, helpful to remember this general principle: When the opponents have an odd number of cards, they will most likely divide as evenly as possible; but when the opponents have an even number of cards, they will most likely not divide evenly. Except that two cards are a slight favorite to divide 1-1.
| 1002. Distribution of defence's cards in a suit | |||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Total cards | Most EVEN split | UNEVEN splits | |||||||||||||||||||||
| split | probability | split | probability | ||||||||||||||||||||
| 2 | 1-1 | 52% | 2-0 | 48% | |||||||||||||||||||
| 3 | 2-1 | 78% | 3-0 | 22% | |||||||||||||||||||
| 4 | 2-2 | 40% | 3-1 4-0 |
50% 10% |
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| 5 | 3-2 | 68% | 4-1 5-0 |
28% 4% |
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| 6 | 3-3 | 36% | 4-2 5-1 6-0 |
48% 15% 1% |
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| 7 | 4-3 | 62% | 5-2 6-1 7-0 |
31% 6% 1% |
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| 8 | 4-4 | 33% ? | 5-3 6-2 7-1 8-0 |
46% ? 15% ? 4% ? 1% ? |
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| No need to remember this chart, just note that with an even number of cards, an even split is the less likely, but with an odd number of cards the most even split is the most likely. | |||||||||||||||||||||||
| Add to your customised cribsheet | |||||||||||||||||||||||