Thursday, April 26, 2012

Two golden rules of bidding

It is a truth universally acknowledged, that a bridge player with a good hand who makes a forcing bid below 3NT must be in want of a game (but not necessarily slam).  This is the well-known principle of "game before slam," expressed as Jane Austen might have put it.  This rule is typically invoked when the partnership hasn't yet determined which strain they will play in at game. Because games are so valuable and so common in bridge, getting to the right one takes precedence over the more remote possibility of slam. In other words, if a bid could be construed as either a probe for the right game, or a cuebid in search of slam, assume the first meaning.

What is not so firmly established in the bridge player's compendium, but which is no less significant in my view, is the following rule which applies to competitive auctions: "declare before game."  This recognizes that it is more important in competitive auctions to play the hand than it is to get to a game.  Or, if a bid could be construed as either a competitive bid, i.e. to play, or a cuebid in search of game, assume the first meaning.  It goes without saying that slams take a distant back seat in competitive auctions.

Here's an example from this week's Swiss team event at the club.  You are South holding ♠ AQ  J74  A653 ♣ K976 (nobody is vulnerable). Partner deals and opens 1♣.  RHO comes in with 2♣ (Michaels) and you bid 2, showing a limit raise or better in clubs.  LHO now bids 3♠!  Before I tell you what partner does next, let's think about the possible continuations.  What would it mean if she bids 4♣?  Apply the rule of declare-before-game.  4♣ would therefore say: "I have extra length in clubs (presumably five or more), a minimum hand and I want to play 4♣ unless you have enough to bid 5♣."  What would double mean?  Well, this obviously depends on your agreements, but we've already established that it's "our hand" and that we have a fit in clubs.  My preference would be for penalties (because of the 2 cuebid) but, given that you are sitting under the spade length, a good alternative is "BOP" (balance-of-power) asking for your opinion as to whether to bid on or defend.  [Do you know what double would be in your partnerships?]  3NT would obviously be "to play."  Any other bid at this level would, I believe, be a cuebid in search of slam committing our side to at least 5♣.  Pass covers every other possibility.  In this case, your side is committed only to 3♣ so partner's pass would not be forcing, even though she's hoping you'll be able to do something. From her side of the table it's possible, though perhaps unlikely, that the best your side can do is to defend 3♠ undoubled.

As an aside, if LHO had bid 2♠ only, a couple of things would change.  Pass in the direct seat would now be 100% forcing because we are committed to try for at least 110.  Three-level cuebids would show stoppers in search of game, not slam.

In fact, partner bids 4♣.  I think it's a toss-up whether you should pass or bid on to 5♣.  You have a lot of losers outside the trump suit.  It turns out that on this board, you can take nine tricks in notrump or eleven in clubs (partner has AQJxx of clubs, AT9x of hearts, and two small doubletons).  But from partner's side of the table, where spades can be profitably led, one trick fewer in each strain is possible.  To give credit to the opponents, the 3♠ call gave our side a real problem.  It was now unrealistic (my opinion) to bid 3NT and so 4♣ is in fact the best that we can do, practically speaking.

So, what happened?  Our side ended up in 5♣ from the wrong side for –50, losing 6 imps (and consequently one VP).  At the other table, where your scribe was playing, the auction was quite different.  North did not open, and South opened 1 in third seat.  After a heart response, the bidding died out in 1NT from the right side.  We didn't take all of our tricks and so we were –180.  Obviously, the swing could have been much greater (10 imps and another two VPs).

Our teammates were not clear on the meaning of 4♣.  In fact, North intended it as competitive, just as the rule would suggest, while South thought that it was Minorwood in search of slam.  I think it's very important to have these two golden rules of bidding well understood and agreed in a partnership, because these types of auction occur quite frequently.

Monday, April 23, 2012

Distribution-showing bids in competitive auctions

I'd like to start this blog post with a definition of three different types of auction:
  • constructive: an auction in which the opponents take no part (may also be referred to as non-competitive);
  • semi-competitive: an auction in which the opponents intervene at a relatively low level and then subside;
  • competitive: an auction in which both sides are active right up to the final decision.
In some competitive auctions, it is clear which side "owns" the hand and therefore is trying to bid constructively, and which side is bidding destructively.  We wouldn't go too far wrong in labeling this type of auction semi-competitive but still it's a fact that even in those situations that we think are clear, it turns out that the side with the fewer high-card points sometimes emerges as the "owner" of the hand.
I've written previously about the use of fit-showing jumps which can be very beneficial in both competitive and semi-competitive auctions.  The tool is primarily used by the side which "owns" the hand and, in particular, by responder.  Like splinter bids, FSJs can describe your
distribution to partner quite accurately in one bid.  Unlike splinters, however, which are at their most useful in constructive and semi-constructive auctions, FSJs are usable only when the opponents have intervened or, by explicit agreement, when the jumper is a passed hand and cannot be showing a strong jump shift.

But there are many other types of distribution-showing bids in competitive auctions.  The particular type that I want to describe here is a kind of sacrifice-control bid which I'm going to call a
sacrifice-discovery bid (SDB for short).  Let's jump right into an example:

In a recent unit game, I picked up a good competitive hand, at favorable vulnerability: ♠ 7  KQT965  43 ♣ KQ94. My RHO opened 1 and I overcalled 1. LHO doubled and partner jumped to 3.  Since he could have made an FSJ, the chances of him also having Axxxx of clubs was non-existent, but he might have some length in clubs even so. In any case, I was fully prepared to save in 4 if necessary so over RHO's 3♠ call, I bid 4♣.  This conveyed the following message to partner: "I know that 3 is as high as you want to go based on the auction so far.  However, would you be willing to go to 5 in the knowledge that I have a useful second suit of clubs?  In other words, do we have a double fit?"

A double fit is typically worth at least one more trick – as good as or even better than the possession of extra length in our suit.  In fact, what happened was that LHO bid 4♠ and all passed.  Sometimes, these bids have unexpected outcomes.  Sometimes good and, to be perfectly honest, sometimes not so good.  However, there was safety in the 4♣ call because partner was, by virtue of his previous preemptive 3 call, effectively barred from any further bidding unless he had good length in clubs.  At worst, he would know that a club return was safe if he happened to win an early trick in trumps, for example.

The main job of the 4♣ SDB had been done – it prevented us from taking what might be an expensive sacrifice over 4♠.  This time, however, we were even more fortunate.  The expert declarer on my right, assuming that I was 6-5 or 5-5 and fearing an early ruff (or two) delayed setting up clubs (dummy had AJ632) until it was too late.  The result was that we went plus defending a contract which, double-dummy at least, should have been made.

Either partner might make a sacrifice-discovery bid but, as in this case, it is most likely to be perpetrated by the original bidder of our suit, since partner will have had a chance to make an FSJ already.  Now, I know of at least one likely reader of this blog that believes that it is a cardinal sin to give away this kind of information to the opponents.  Yet, unless an opponent happens to have a stack in the second suit, he or she will generally not know if the double fit condition exists or not.

Perhaps my LHO would have done better first to double 4 and then, when we ran back to 4, assuming that her partner had not doubled that, bid 4 or, conceivably, double again. They would have earned 100 for doubling us in 4.  Not as good as the 3 that they could easily make nor, obviously, the 4 that they should make with double-dummy play.

Of course, sometimes the opponents don't give us the room for such a bid, especially when our second suit is clubs.  But count me as a fan of the method, whenever I find that it is available.

Tuesday, April 17, 2012

Penalty Doubles (part 2)

Last time we talked about the five types of penalty double:
  • Lead-directing;
  • Equity-preserving;
  • Penalty-increasing;
  • Psyche-exposing;
  • Tactical.
In particular, we discussed lead-directing doubles in general terms. I mainly want to talk about equity-preserving doubles now so let's get the other three types out of the way quickly.

We'll start with the least common type of double: the tactical double. There are probably many different types of tactical double such as doubling cuebids to sow doubt in the minds of the opponents as to the placement of key honors. But surely the best-named toy here is the Stripe-tailed Ape double whereby, as the opponents bid on their way to a slam you think they might make, you deliberately offer them the option of playing a lower-level contract doubled. When they expose your ploy by redoubling, you have to run like the proverbial ape. Several things can go wrong of course. You'd better have a good suit to run to when they redouble because you are likely to be doubled. Or, the slam might not be making because of some holding in partner's hand, and instead of going, say, +100, you are now -990. Or what happened to me several years ago at a club game playing against an expert pair. I doubled 4♠ and didn't get redoubled. The hand made 6♠ and so I was -1190. But it was a Pyrrhic victory – nobody else actually bid the slam so I got the same bottom as I would have had my opponents bid the slam (-1430). And, who knows, maybe they weren't going to.

In the early days of bridge, when psyching was very common and there were no take-out doubles, the primary reason for the double was, I would argue, to expose pyschic bids. Without the double, the opponents could just bid your suits and make it impossible to find your games or slams. Psyches are relatively uncommon these days. But because we've traded most of our penalty doubles for other kinds of double, a psyche can be hard to expose and consequently quite effective.

Hand-in-hand with that use of double, came the penalty-increase usage. Overbidding was common and double was necessary to keep everyone reasonably honest. The idea is of course that if the opponents have bid a contract (usually a game or slam) that we are pretty sure is going down at least two tricks, we should double, providing of course that our double doesn't tell declarer how to play the hand successfully. Opportunities for this kind of double don't come up all that often. The standard of bidding these days is sufficiently good that, in a non-competitive auction, if a pair bids a game, they will have a play for it unless there are very bad breaks which could not be anticipated. We need to have well-placed cards, especially trumps in a suit which, preferably, declarer has bid more or less on his own, and some sure tricks like aces. The reason for all the caution is simple arithmetic. It's madness to try to increase a penalty by 100 if there's a chance that they might still make (or, worse, make because of the double). Assuming the more favorable case where our double makes no difference to the play, we risk 170 points (for a major suit game) to gain 100. If they redouble, then we are risking 340 to gain 100. For a slam, the odds are even worse. But if the double tips declarer off to the winning line in a game contract, we are now risking 890 points for the sake of 100.

Note that there's another more subtle reason why the double might not pay off. If the trump stack and the outside high cards are all in your own hand, you are quite likely subject to repeated endplays. It's much better if the trump stack is sitting over declarer and the high cards of the partnership are in the other hand. In my partnerships where we play a two-way double scheme, most of our best penalties come from partner passing a cooperative double for penalties – precisely because now the partnership assets are likely to be split. In general, in a non-competitive auction, if the opponents get too high, it is generally going to be good for us with no need to gild the lily.

Now, we come to the equity-preserving double. Essentially, the opponents have sacrificed (deliberately or accidentally) over your contract that you were expecting to make. At the game level, the equity-preserving double is probably at least as common at IMP scoring as at matchpoints. At levels below game, it is much more common at matchpoints. Let's take an obvious example. Red on white, you pick up a balanced 15 count and open 1. Partner makes a limit raise of 3 and you bid the fourth heart. One of the opponents then bids 4♠ and you double. You were very optimistic that you were making 4 but both you and partner have balanced hands (he would likely have splintered with a singleton). You therefore double and hope to earn 800 which would more than make up your "equity." At IMPs, you would likely gain 4 imps and at matchpoints you will probably get close to a top. On the other hand, if you can only take 6 tricks (quite likely), you will lose 3 imps at teams (not a tragedy) but will likely be well below average at MPs. But you did what you could to preserve your equity by making the penalty double. You haven't suggested a stack of trumps or that suits will be splitting or lying badly. You are just doing what you must – "take the money."

Playing matchpoints, you have to be much more aware of possibilities for an equity-preserving double. Let's say you and your partner have bid to 3, a contract which you feel fairly confident of making as you have the balance of power and an eight-card fit. Your vulnerable opponents now bid 3♠. If your hands are sufficiently balanced that you don't expect much play in 4, you double. Assuming that you get them down 1, you will earn 200 which will more than make up your "equity," the 140 you could have scored on your own. On a 12 top, you can probably expect to earn 10 for +200, 6 for +140 and only 2 for +100. If it turns out that they (and you) can make 140 all along, you were probably getting only a few matchpoints for -140 so if you slip and they score up 530, you could say that the double (combined with bad defense) only lost a few matchpoints.

Note that the equity double doesn't work so well if the opponents are non-vulnerable. You would have to beat them two to make up your equity and if you're sufficiently strong for that, there's a reasonable chance of successfully making 420 or 620 in game. That's because the most common number of total tricks in these competitive deals is 17. If you can beat them two in a nine-trick contract, you might well be able to make your own ten-trick contract. Every hand is different, obviously, and each situation has to be evaluated on its own merits. But if they are vulnerable, you only need to double and collect a one-trick set for a result which will at least beat all other part scores.

Equity-preservation can also result from passing a two-way (or cooperative) double.

The main point is that your double in these situations is not intended merely to increase your penalty. At matchpoints, for example, when your opponents have overbid in a non-competitive auction, you will be getting a good board anyway. Doubling is unlikely to give you more than one or two matchpoints extra and there is a risk that you might lose what would otherwise be an average board if they do in fact make the hand. This is especially true when the opponents overbid to four or five of a minor. Even if they make, many of your virtual teammates might manage to make 3NT, possibly with an overtrick. If they don't make, a plus score should be good for you – you weren't in the auction to begin with.

Now, to the quiz in part one. Your hand was: ♠83 QJ98 62 ♣J9754, red against white. The auction proceeded, starting with partner: 1–2–p–2NT; 3–4–4–p; p–5–p–5NT; p–6♠–p–p; X–p–p–p. What did you lead? If you led a club, you are the hero! A double in such circumstances never asks for a trump so that's out. Partner has freely bid two suits and the opponents seem to be unimpressed. Double can't help distinguish between partner's two suits (without the double, my rule is always to lead the second-mentioned suit because that is generally the suit for which partner went furthest out on a limb). So, in keeping with the rule mentioned in part one, partner doesn't want you to lead either of his suits. Partner ruffs the club and cashes one diamond for +100. At the other table, it went p–2♣–p–2; X–2♠–p–2NT; p–4♠–p–5♠; p–6♠–p–p; X–p–p–p. Opening leader, apparently not familiar with the central concept of part one of this blog, led a diamond for -1210.

Sunday, April 15, 2012

Penalty Doubles (part 1)

I've written a lot in these blogs about cooperative (two-way) doubles which are primarily intended to discover shape information.  Quite often, these are converted to penalty doubles by passing.  What I want to talk about here is the true penalty double.  This is the first of two parts.

I've identified five types of penalty double: (1) lead-directing, (2) equity-preserving, (3) penalty-increasing, (4) psych-exposing, and (5) tactical.  Let's start with the first type, the lead-directing double.  There are two sub-types: (a) double of an artificial bid and (b) double of the final contract.

Because the first type occurs before the natural conclusion of the auction (at least you think it's not yet over), it can serve both as a lead-director and as a proxy bid, suggesting values in the suit doubled.  Partner might use this information to sacrifice later on or, if he ends up on lead, then he will normally lead the suit doubled.  One of the advantages of this type of double is that it's "free".  But we all know that there's no such thing as a free lunch!  I learned this the hard way many years ago in my first Flight A Swiss.  Playing a pair of Grand Life Masters (yes, I should know better), I doubled a Stayman 2♣ bid for a club lead.  LGLM redoubled and RGLM passed.  There was no way to defeat 2♣ and one of those special scores got burned into memory: 760.  This was "only" a 4-imp loss as our teammates made 600.  But overtricks, if there had been any, would have been exorbitant – additionally 7, 3, 2, etc. imps.

But I digress.  The second type of lead-directing double is when the (expected) final contract is doubled to ask for a specific lead.  First introduced back in the early days of bridge by Theodore Lightner, this double is intended to create a penalty where none existed before.  In other words, if you make an unusual lead, partner, this contract is going down, otherwise, it will be making.  Because this type of double doesn't simply increase the penalty but instead defeats a contract, it is generally the most profitable of all doubles.  Because the margin of error in slams is so small, the Lightner double was first described as a defense to slams.  Imagine the opponents have bid to 6♠ while you hold ♠A2 and a side void, you know that the slam is going down – if only you can persuade partner to lead your void suit.  It is of course just possible that declarer has a void in the same suit and your plan may go awry but that would be unfortunate to say the least.

[In a distant aside, that actually happened to my opponent in a bizarre incident that occurred just before I gave up bridge for 18 years.  The scene was the 1980 NABC in Detroit, my first tournament of any kind, soon after I'd learned the game.  I was in 7 which was doubled for a diamond lead.  The opening lead was a diamond and I claimed (there were approximately 15 tricks on view and I was void in diamonds).  The director was called and it was ruled that, because I didn't explicitly say in my statement that I would over-ruff my RHO, I was down one.]

Back to the present.  Again, there are dangers with lead-directing doubles.  The most likely thing to happen is that one of the opponents will pull the contract to no-trump.  Now, your precious ruff just evaporated and if they can still manage 12 tricks you will have helped them go from a failing contract to an impregnable one.  Your partner will undoubtedly point out that he was going to lead your void suit without your silly double.  The next danger is that partner won't know which of the other three suits to lead.  If he is long in dummy's suit, that should be pretty obvious.  But dummy doesn't always oblige by bidding a suit.

This is where experts invoke the following rule: if you have bid a suit during the auction (or made an earlier lead-directing double) and you subsequently make a lead-directing double of the final, freely-bid suit contract, you definitely do not want partner to lead your suit.  You were expecting that lead and there would be little point in doubling simply to increase a penalty that was already coming.

Of course, there are the usual dangers.  You might be incorrect about your ability to defeat the contract.  Partner might not recognize the difference between a lead-directing double and one of the other two types of penalty double.  One of these situations arose recently in a pairs game.  I should note in passing that the strange mechanism of scoring used in pairs games complicates the entire business of penalty doubles. I held ♠ – 952 AJ7432 ♣ AQJ5.  My LHO opened 1, partner passed and RHO bid 3 showing a four-piece constructive raise of hearts.  Assuming my opponents were bidding honestly, that left partner with at most one heart.  I wanted to show my diamonds, force the opponents to game, make a lead-directing double and hope that partner could work out to lead spades (and come up with a fourth trick).  In retrospect, I'm not at all sure why I was so optimistic!  Since we play two-way (cooperative) doubles, I had to trigger penalty doubles with either a lead-directing double of the 3 call, or make a bid that showed a good suit (similar to a preempt) with 4.  I chose the latter (which, had I bought the contract, would have been down 1).  LHO obliged with 4 and it came back to me.  Now to spring the trap (so I hoped).  I doubled confidently and awaited a spade lead.  I got a diamond.  There was still a chance though.  I won the ace, cashed the ♣A and led a low club to partner's ♣K.  At this point, a passive lead would have resulted in down 1.  Unfortunately, partner tried to give me a club ruff.  There was now no way to defeat the contract and so we went from a potential +800 (this would require a spade lead, plus a double-dummy defense of me leading a club to partner's T at trick 2) to -790.  Quite a swing! 

We had some friendly discussions about which would be the better club to continue at trick 3, whether it would have been better to double 3 or bid 4, and so on – but it is nevertheless an instructive hand.  The essential problem was that it wasn't clear to partner which type of penalty double I was making.  I think the only contenders, based on the auction, were types 1 and 3 (lead-directional and penalty-increasing).  Had I doubled 3 then I think type 2 (equity preservation) might have been possible also.

Now, here's a test to see if you've been paying attention.  This hand actually occurred today in the seventh match of a sectional Swiss.  Your hand is:  ♠ 83 QJ98 62 ♣ J9754, red against white.  Hold on to your hat.  You won't experience this auction everyday!  Starting with partner, it goes 1–2–p–2NT; 3–4–4–p; p–5–p–5NT; p–6♠–p–p; X–p–p–p.  What do you lead?  Answer in part two.

In the next part of this series, I will try to show that, while penalty double type 3 (penalty-increasing) may be the traditional interpretation, the species is actually less common at duplicate bridge than one might expect.

Thursday, April 5, 2012

Using double to find out about fit

Lately, I've been looking for a simple, unifying principle for the system of two-way (a.k.a. cooperative or DSIP) doubles which I've been promoting in these blogs.  If you're a regular reader, you'll know about my set of triggers which switch the meaning of double from takeout to cooperative and from cooperative to penalty.  A hand came up the other day which is a perfect example of using double to find out whether we have a good fit.

Playing with my GNT teammate Jay in an ACBL speedball tournament online (IMPs), I picked up this hand: ♠ –  JT93  T8743 ♣ KQJT.  Everyone was vulnerable and partner opened 1♣.  This was overcalled 1♠ on my right and I made a slightly pushy negative double.  LHO now jumped to 4♠ and this was passed back to me and I had some thinking to do.  First of all it seemed that we had at least half the deck, high-card-point-wise and possibly even the balance of power.  We play a strong no trump so it was possible partner had a weakish balanced hand.  On the other hand, playing any sort of notrump, he could have a reasonably shapely hand with clubs and somewhat more than a minimum.  He doesn't know that my hand has a good club fit, after all.

I decided to get very pushy, and a bit undisciplined given the void in spades, and doubled.  This was no doubt due, to some extent, to having enjoyed a beer earlier in the evening.  Partner's hand was ♠ AJ3  AQ84  Q96 ♣ 974.  He was happy to pass and we set 4♠ by one trick for a gain of 7 imps.  Had he been dealt a hand like ♠ J4  AQ8  Q96 ♣ A9743, then he would pull to 5♣ and either they would bid on to a making 5♠ or we would be down one in 5♣X.

At least, that's what should have happened.

In reality, not having had a chance to discuss this sort of auction, I was worried that my double might be interpreted as penalty.  Of course, with partner's actual hand, everything would work out just fine.  But what if he had a small doubleton spade and five clubs to the ace?  We could easily suffer a double-game-swing.  Therefore, I bid 5♣ instead.  Partner pulled this to 5 and played it there.  The defense was imperfect and we ended up not only undoubled but down only two.  This was good for a 1.25 IMP loss on the board, almost a push.  Other, less fortunate pairs played 5, going down three doubled and losing 11 IMPs.  That's a total swing of 18 IMPs!!

Interestingly, of the 83 times the board was played, the scores ranged from -1100 to +500 with only eleven pairs actually making a contract (nine in 3♠, two in 3).  25 pairs played doubled contracts one way or the other, all failing.  Despite the void in my hand and 18 total trumps, there are only 17 total tricks available on this deal.

What follows is a long theoretical discussion.  Feel free to skip it :)

The basic issue is fit.  If you are considering outbidding your opponents in a competitive auction, by which I mean where each side has approximately half of the high cards, you are going to feel much more confident when your side has established a "good" fit.  While this may sound somewhat arbitrary, I'm going to define a good fit as nine cards or better (or two eight-card fits).  In the type of auction I'm talking about the "law" of total tricks is more relevant than in other situations.  And since most of these competitive decisions will involve bidding at the three or four level, a nine-card or better fit is going to give you a fighting chance of either making your contract, or at least not being penalized more than 100.

According to a pet theory of mine (actually a corollary of "the law"): if each side has exactly half the high cards, and if the deal is distributed in a reasonably "pure" and appropriately shapely manner, then each side will be able to take the number of tricks equal to their trumps.  On average!  Since this is a corollary of the LOTT it is certainly no more accurate and in practice it will be even less accurate for any particular hand.  A queen more or less will, again on average, make a difference of one trick either way. 

That's why a nine-card fit is so useful for these competitive situations.  A reasonable alternative to a nine-card fit, by the way, is a double-fit of two eight-card suits.  We're coming to the point soon, I promise.

In a perfect world, a competitive auction is won by the side which has the majority of the following assets/attributes:
  1. the higher-ranking suit (asset);
  2. the balance of power (attribute);
  3. non-vulnerability (ditto);
  4. an established good fit (ditto).
 So, let's take as an example, an auction in which the opponents have bid 3.  You're not sure if they have a "good fit" but that's not your concern.  You're considering your options: do we have the higher-ranking suit (in this case spades)?  This is the only "asset" on the list: it cannot apply to both sides as the others can. Do we have the balance of power?  It's hard to know this for sure but it should be possible to make a statement such as "with high probability we have at least 20 hcp."  Are we non-vulnerable?  This is as simple as looking at the board.  Do we have an established good fit?

If all of the answers are yes, then I think we can safely bid on.  In fact, we might want to consider bidding 4♠ any time we have 1, 2 and 4.  If we have 1, 3 and 4 then we can probably bid 3♠ with reasonable safety.  What if we have 2, 3 and 4.  This will involve bidding a part-score at the four-level which is not one of the choicest contracts in bridge.  We might want to consider a penalty double if we have the right holding, assuming that we really do have the balance of power.

Now we come to the crux of the matter.  What if 1, 2 and 3 pertain?  We have the spades, we think we have the balance of power and we are not vulnerable.  But we don't know if we have a good fit.  We might have a fit, but we don't know how good it is.  The vulnerability plays a big part in the decision.  If they are vulnerable, we are somewhat more predisposed to defending doubled and if we are vulnerable and they are not, especially if we're not sure we have the balance of power, then we are more prone to pass.  How can we tell if we have a good fit?

We double, cooperatively, showing shortness in the enemy suit, typically a doubleton at lower levels and a singleton at higher levels.  This shows a moderately shapely hand with support for any unbid suit and tolerance for partner's suit (if any) that we have not raised. If partner has wastage in the enemy suit and/or a very balanced hand suitable for defending, and no undisclosed length in our suit(s), then he will convert the double into penalties.  If it turns out we have a double fit or extra length in any known fit (if there is one), partner will pull the double.

Let's now consider the situation where we do have the balance of power, but not the rank advantage.  Let's say, too, that both sides are vulnerable.  If we have already established a good fit, then there is no need to use double cooperatively.  We can now use double for penalties! See the DSIP Rule Summary.  For other blogs on this topic see, for example, The Dead Auction Rule, Inferences from Two-way Doubles, Doubling Intervention After We've Found a Fit.