We know a 9 card fit is a good thing. There are various theories to explain why including something called "the law of total tricks". I want to give a practical guide of what to do with a 9 card fit.
1. When you have a flat hand, don't get carried away
9 Card fits might make little or no difference when you have a flat hand. In such situations you can be slightly encouraged (imagine the 9 card fit being worth about 1-2 points), but basically not do much different.
2. When you are distributional - 9+ card fits can make a big difference
Take this hand for example
On this auction lets say partner showed 4 card support and 6-9 points. I think it is pushy but not terribly unrealistic to bid game. This Partner had Kxxx spades, they broke, and the heart finesse made, and declarer was able to ruff hearts. Declarer made an overtrick opposite 6 points!
There are numerous problems with a hand like that opposite 3 card support. In an 8 card fit lots of things can go wrong including
- Declarer will be forced to ruff the side suits, and this could cause declarer to lose control of the hand.
- It is harder to draw the enemy's trumps, they have more of them, you have less of them.
- The trump suit is too weak, opposite 3 cards there is likely 1-2 trump losers even if they break well.
- Setting up that heart suit is harder to do when partner only has 3 trumps, you may not be able to ruff as many times in dummy.
Overall High card points are much more relevant with an 8 card fit.
Whereas
with a 9+ card fit, distribution and long side suits are paramount. Things like voids and singletons are working overtime when you have a big trump fit, whereas they are not as strong when you only have an 8 card fit, unless you have plenty of high card points.
Trump quality
Trump quality matters a LOT less when you have a 9+ card fit. Take this hand for example
Lets be fickle and this time say that partner's 3♠ raise shows 0-6 points. Do you bid on or not? If you look at your side suits, they are actually really good. You may only lose 1-2 tricks in total in the side suits. The big problem is the trump suit, however when partner has 4 cards, we hold our losers to a maximum of 2 if they break, and sometimes less for example if partner has the Ace or the King of trumps.
overall - bid on with an 9 card fit, because trump texture no longer is a problem, whereas with an 8 card fit - its likely that you have too many trump losers to make game opposite a weak hand.
In summary my recommendations are
- When you have a distributional hand, bid at least 1 more level with a 9+ card fit than you would otherwise
- With a flat hand, don't get carried away
- Don't worry about a bad trump suit when you have 9+ card fit, but DO worry about it when you only have an 8 card fit.
Read next: Hand Evaluation: Do Less with a Weak Trump Suit · Hand Evaluation: Do More with a Good Suit · Loser Count for Bidding: Judge Contracts Better