The logic behind LTC is that provided you've found a trump fit (at least 8 cards), you cannot lose more than 3 tricks in any side suit. Neither can your partner. The reason for this is that any longer cards can either be established as winners or ruffed in the other hand. There are some assumptions behind that but for normal hands, it works fairly well.
Thus anything that doesn't lose, wins. Hence, start off with 24 winners, and subtract up to 3 losers for each partner in each suit.
Assume for a second that you and your partner are in a spade contract, both holding
♠xxxx
♥xxx
♦xxx
♣xxx, where 'x' is any card 9 or lower. Thus you both have 12 losers, and 24 - (12 + 12) = 0 tricks, which is how many you have (the defenders can hold you to 0 tricks by playing long plain suits after the 9 side tricks have been taken, enabling them to score
♠J, Q, K, A separately at the finish).
As you can see, you will lose 3 tricks in each side suit and 4 trumps, or all the tricks.
Now change it so that you have
♠xxxx
♥xx
♦xxxx
♣xxx and partner has
♠xxxx
♥xxxx
♦xx
♣xxx. You've removed a loser from each hand, and now you can take 2 tricks: a ruff of the 3rd red card in the opposite hand. 24 - (11 + 11) = 2 tricks.
Give either player an ace, and the losing trick count falls by one. replace 2 xx's with an AK, and it falls again by another one.
Where LTC breaks down most often is in the following situations:
(1) You are in an insufficient trump fit. Say, something like 5-2 trumps, where the two in dummy are small and can be overruffed when you attempt to ruff losers there. This can lead to dummy's trumps being drawn by your opponent, or your losing more trump tricks than expected. (This can easily happen in a 5-3 fit as well, especially when the 3 are small!)
(2) You are in any misfit.
(3) You are in NT (the LTC is pretty much useless here).
(4) You are armed with many controls, sufficient trumps, and a long, running side suit (the defenders don't have time to get at all of your losers; this is a major reason why Aces tend to be worth much more than 4-3-2-1 point count suggests).
(5) You have unsupported honors and either can't finesse them (entry or timing problems), or the finesse fails.
(6) Key suits break badly (trumps, long suits, suits that require some ruffing, etc).
(7) Your honors are well-positioned behind, or poorly positioned in front of, a strongly bidding opponent.
(8) You have duplicated values. This is by far the most common failing of LTC when a sufficient trump suit has been found, in my experience. Some examples:
(A) Small doubleton opposite small doubleton. Within that suit, you start with 6 tricks and subtract 2 twice, for 2 tricks. This suit provides no tricks, not 2! If it were xx opposite xxxx, you could ruff 2 small ones for 2 tricks. This is an example of DUPLICATED SHORTNESS. Mirrored distributions, where you and partner hold the same length in every suit, are the absolute worst.
(B) Small doubleton opposite small singleton. Again, duplicated shortness. The same is true of shortness opposite a void.
© Minor honors opposite shortness. Like the example from a previous post, KQJx opposite x is a big problem. Both players would be counting on only 1 loser in the suit, distorting the number of tricks their side has available. Unless you have time to establish the QJ (and maybe even the K) for discards in another suit, the honors here are worthless in a trump contract. This is an example of UNNECESSARY MINOR HONORS.
(D) Trump suit duplication. Say you hold ♠AQxxx and your partner, ♠KJxxx. The jack in this case is worthless, and in many cases, so is the queen. They'd probably be more useful in some other suit that needs filler. (An exception: these duplicated honors will be very helpful if playing a contract on cross-ruffing lines.)
(E) Side suit duplication. This is the same concept as trump suit duplication, except only in a side suit in which your side has 8+ of the cards. Side suit duplication is especially problematic if the suit is evenly divided between the two hands and the trump suit is not. (If that's the case, it usually is much better to have the evenly divided suit as trumps and the other suit as non-trumps.)
Many of these problems can be diagnosed or anticipated (or, at least, the
possibility that these problems may exist can be diagnosed) during the auction itself. For example, if LHO opens 1
♥, your partner doubles for takeout, and you hold
♥KQJ7, you can be sure that the heart suit contains duplicated and therefore negative values. LTC will overestimate your playing strength. (Try to bid NT for this exact reason.) Or another example is when partner responds to your bid with a splinter bid. In that suit, Axx(x) and xxx(x) are excellent holdings, while KQJT is terrible, and anything else is somewhere in between.
To continue this point, there are a variety of adjustments that need to be made for minor honors, gaps, etc. For example, Kx is technically only 1 loser, but if your partner doesn't have the Ace or Queen, half the time the king won't take a trick because the Ace swallows it and doesn't promote the Q in the process. Here are some ideas:
- Qxx should be treated as 2.5 losers.
- QJx should be treated as 2 losers.
- QTx ... it really depends on who has the jack, doesn't it?
- AQx should be treated as 1.5 losers.
- AJT should be treated as 1 loser unless you're certain LHO has substantial strength in this suit. (Even then you might be able to force him to lead it.)
- AQT should be treated as 1 loser.
- AQJ should be treated as 0.5 losers.
- Kx is 1 loser unless you know your partner is very weak (then it's 1.5).
- Qx, Jx, Jxx, JTx, Tx, Txx --> these are 100% losers. 2, 2, 3, 3, 2, and 3 respectively.
- AJ is 1 loser.
- AQ is 0.5 losers.
- KJ(xxxx) is 1.5 losers in most cases, not 2 (especially if you also have the T).
- If partner is known to have a singleton opposite your minor honors, mentally make the appropriate adjustments. Bid much more conservatively.
- Trump hands that require some degree of ruffing in one or both hands: try to determine if the length in the suit being ruffed is with the defender to the RIGHT of the long suit (i.e., "ruff right"), or to the LEFT of the long suit (i.e., "ruff wrong"). So, for example, if you and partner are going to play a spade contract and you are long in diamonds, a diamond overcall by LHO is potentially a problem but a diamond overcall by RHO is welcomed! This is especially important when all of your trumps are not high, and can make the difference of anywhere from 1 to 4 tricks in your trick total.
It may seem daunting at first but you can learn to make these adjustments when necessary and in my experience, intelligently adjusted LTC is an effective weapon to figure out what level you should be playing at. For me, starting with LTC and then overlaying some of the concepts laid out by Mike Lawrence in two of his books (
Hand Evaluation, and
Judgment) was the beginning of a new level of sophistication in my bidding practices. I started being able to "see the hands" mentally sometimes, at least to some extent, before the auction had finished.
You will be wrong some of the time using LTC, but you'll be far more accurate than using strictly HCP and length/shortness points alone.