The Law of Total Tricks is more of a competitive bidding guideline than a law.
The Law of Total Tricks advises that when both sides have a fit, the total number of tricks available between each of the two partnerships playing in their trump fit is equal to the sum of the trump fits in both partnerships. For example if North-South had an 8-card spade fit, and East-West had a 9-card heart fit, the sum of the tricks for North-South playing in spades and for East-West playing in hearts would be 17 (8 card spade fit + 9 card heart fit).
How many tricks each contract actually makes isn't solely based on trump length. It depends on a variety of factors including:
- the position of honors
- whether a partnership's honors are working (helping to take tricks, as opposed to be wasted opposite partner's shortness)
- ability to benefit from short suits
- transportation between hands
Even so, the Law of Total Tricks provides a useful estimation of how high to compete. In general, it suggests competing to the trick level of your trump fit.
- With an 8-card fit, compete to the 2-level
- With a 9-card fit, compete to the 3-level
- With a 10-card fit, compete to the 4-level
The Law of Total Tricks doesn't replace judgment. The form of scoring, the vulnerability, working honors, shortness, defensive potential, offensive potential, alternate contracts, etc. should be considered in addition to the trump fit.
The popularity of the Law of Total Tricks has helped players recognize the upside of competing and preempting more aggressively with a good fit, even with minimal values.
When applying the Law of Total Tricks, don't distort the strength of your hand. If North's jump raise to 3♥ shows an invitational raise with 4+ hearts (the standard meaning), don't jump to 3♥ with a weaker hand because of the Law of Total Tricks. If you have a hand like ♠72 ♥Q864 ♦KJ93 ♣Q62 as North, respond 2♥ to show 3+ hearts and 6-10 Support Points. If the opponents later compete to 2♠, you can take the push to 3♥ based on the Law of Total Tricks.
See the following articles to learn more about the Law of Total Tricks: