The Law of Total Tricks is an advanced valuation approach that is very useful in making successful competitive bidding decisions when both sides have a fit. This is not a “Law” in the scientific sense (in science, a Law is correct 100% of the time); instead, it’s a general principle that wins matchpoints very often.
Here it is: The number of Total Tricks (that is, the tricks that each side can take in their respective best trump suits) tends to be the same as the number of Total Trumps (that is, the combined trumps in the best EW and NS trump suits). Here is an example that will introduce you to the Law.
Say that you expect your side to have eight combined diamonds and that you expect the opponents to have eight combined spades; that would be 8 + 8 = 16 Total Trumps, and the Law suggests there are also 16 Total Tricks. The Law suggests that if you can take nine tricks with diamonds as trumps, the opponents can take 16 – 9 = 7 tricks with spades as trumps. Similarly, if they can take nine tricks in spades, probably you can take seven in diamonds. The other likely situation is that each side can take eight tricks. We find that this sort of evaluation helps to make better competitive bidding decisions.
For more information see Chapter 2 of Real World Bridge 4: Competitive Bidding Part One.