Finding a fit with partner relies on knowing how many cards they have in the suits you bid. You can generally discover a fit quickly when opener starts with a major - for most players that shows 5 cards and we can support their suit with only 3 cards.
It’s a bit more difficult when after you open a minor, responder introduces a major. As we all know, this only promises 4. She could have more, but she may not, and if you are sitting there with 3 cards in her suit, then it would be great to know if there’s a sneaky 5th card in her hand to guarantee your fit.
Sometimes the auction allows responder to rebid her major, promising that extra card, but that’s not always possible. Fortunately we have other tools at our disposal, and one of these is the support double. Here is a good example of its value. This comes from board 17 of the Two Men and a Truck Intermediate and Restricted Swiss Pairs - Session 2 on Sunday June 21.
North opened 1D and South responded 1S. West overcalled 2H and N decided to show his second suit, bidding 3C. From South’s point of view there was clearly a fit in clubs, and with 11 total points, game would likely be on. But game in a minor is not straightforward, so he invited with 4C and that’s where it stayed.
The lead was 3H which Declarer won with the A. There were two ways to play the hand, one involving cross-ruffing, and one involving capitalising on that nice spade suit. Declarer did a good job to choose the latter. He drew two rounds of trumps and then started on the spades. West was able to take their winning club when they chose, but other than that and the A of spades, there were no other tricks for the defense. In total Declarer made 11 tricks, being 4 spades, 1 heart, 1 diamond, 3 clubs in the N hand and 2 diamond ruffs.
Eagle-eyed observers will have observed that 4S was also cold and an easier contract, but how to find it? The answer is that after the 2H overcall, N should have made a support double. This type of double is made by opener, and promises exactly 3 cards for responders major, which would have been great news for S who would have known about the fit in spades. He could show his invitational hand by jumping to 3S, and then N would accept it and they would all live happily ever after.
You can forgive N for forgetting about this gadget. He has a nice two-suited hand, has shown the first one, and by introducing the clubs, he gets to communicate 9 cards - as well as the 4 clubs, he is now marked with at least 5 diamonds in our system. But the most important cards were the 3 spades, and those stayed hidden, leading to the wrong contract.
The best auction therefore would look like this: 1D - 1S (2H) X - 3S - 4S. You can see the obvious value of the support double, but of course it does rely on the opponents interfering. Despite what one of my partner’s tried to do to me a week ago, you are not actually allowed to double your partner.