In modern bridge, the majority of doubles are used as takeout or negative - meaning something other than penalty! Many years ago it was different, doubles just meant - I think you've overbid and you're going down!
So how do we penalise then?
For starters, one of the most common ways to penalise is for partner to make a takeout double, and for you to leave it in. This is a typical example
Partner has opened the bidding and the opponents have bid 1♠, vulnerable. You would love to snap up a big penalty here, but you play negative doubles here showing 4 hearts.
You have 5 spades, the overcaller has 5 spades, partner will always be short, typically 0-2 spades, so will have the correct shape for a takeout double.
Lets put you on the flip side of this
Up to you - are you going to pass, rebid your clubs or .... double?
When a bid is made on your left and followed by 3 passes, its very possible that partner's pass was a hand that wanted to penalty double. Typically consider cooperating and doubling rather than bidding - partner is hoping you will!
The correct bid on the above example is double.
Lets look at basically the same idea
You have a minimum 12-14 style opening bid, nothing extra - is it appropriate to just pass? This is the type of auction where double is almost mandatory - a bid on your left followed by 3 passes, there's a good chance partner has a penalty double hand sitting there, but is unable to strike!
A note on ethics
Lets use that above auction, and think about what you would do here.
Partner is an opening hand, you have great hearts, the opponents are in for some serious trouble, a blood bath - you have 5-6 tricks in your own hand probably, that's before partner gets involved.
You cannot pause and pause, thinking about the situation for a long time, licking your lips, and then gradually pass, stare at partner and prompt him to make a bid.
You need to pass in tempo, as if there was nothing to think about, as if you had a boring hand, as if the opponents were not in serious trouble.
IF you take 10 seconds to pass instead of 3 or 4 seconds, you alert partner to what is going on - and your partner now basically cannot double. This is both a rule of bridge and also an important ethical point.
It is not always easy to pass in tempo, sometimes we need to think and remember what to do. If you find yourself thinking for say 5-10 seconds or more, do not pass and expect your partner to bid now, no further bidding will be allowed basically - rather just bid 3NT or something sensible.
This is a complex topic, passing in tempo, and one that I am only touching on here, there's endless discussion on this, but the rules are clear about it.
Pure penalty doubles
When is a double actually penalty? This is a notorious area of partnership confusion. There are a few solid rules that we can rely on
1. Double of Natural No Trump is for penalty
So for example
This is simply a penalty double
Again this is a penalty double by partner.
2. You had the chance to double earlier for the exact same meaning, and did not
This is worth discussing with partner, although its common its not 100% standard.
This double by South is penalty, why?
If south wanted to make a takeout double of spades, it could've been done at the first turn (doubling 1♠). Since that was not done, South no longer can have a hand that is "takeout of spades". Instead, South has a pure penalty double - a good hand with good spades, say 15+ with good spades.
3. X, X, X - the third double is penalty
A common rule is that we have three doubles, the first two are takeout, the third one is penalty. For example,
Here, the first two doubles were takeout and negative (often we call a double by responder a negative double instead of takeout). However, the double of 2♣ is penalty.
4. Doubles of high level auctions
These aren't penalty in the sense that we are expected to have a lot of the opponents trumps, but often they will be left in for penalty, when partner has an ordinary hand. For example
Both doubles were takeout, yet most of the time we just defend on the 4 level or higher, rather than risking the 5 level (we normally want very distributional hands to venture out to the 5 level, if both players are reasonably flat its likely that defending is better).
5. Following a redouble
If your redouble is natural, showing a good hand, its common to play penalty doubles to follow. For example, the standard interpretation of this
Here we are saying that we have 10+ points, and the next double is penalty. Both sides can contribute a penalty double to whatever they land on.
In modern bridge there are a lot less pure penalty doubles than many years ago. Overall it is better and more common to have a takeout double available - one of the most precious competitive bidding tools. Getting used to the idea of passing in tempo is one of the major ways you will penalise your opponents (your partner will often reopen with a double).
More on doubles: Takeout Doubles in Bridge: The Complete Guide · Negative Doubles in Bridge: What Responder's Double Shows · Support Doubles in Bridge: How and When to Use Them · Four-Level Doubles: What to Do Next · High level doubles - penalty, negative, takeout?
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