A double in bridge bidding can be either a takeout double, used to ask partner to bid, or a penalty double, used when a player thinks the opposition won't make their contract.
How can I tell whether a double is penalties or takeout? The state of the auction determines whether a double was meant for takeout or for penalties.
How can I double on the Sky Bridge Club computer? The computer a double is shown as an 'X' in the bidding area.
X means double
Here's a perfect hand for a takeout double. Notice that South has length in all of the unbid suits so that whichever one North chooses will be fine.
A takeout double
This next hand is not suitable for a takeout double. South has plenty of points but without length in spades it's better to defend. Partner's always have the habit of choosing your worst suit when they respond to your double so make sure you have at least 3 card support.
Pass! - it's ok to defend sometimes.
Double then Bid A takeout double can also be made with 16+ points, even without support for the unbid suits. A simple overcall in bridge shows a maximum of about 15 HCP but you can double and then bid your suit to show a stronger hand. A takeout double normally shows support for all the unbid suits and whatever suit partner responds with should become the trump suit. That's the whole idea of the takeout double in the first place! - get partner to choose trumps. If, after hearing partner's response, the player who doubled bids a new suit then the implication is that player doesn't have support for the unbid suits after all and therefore must have a strong hand.
If partner doubles then you must bid, even with a weak hand. Bid your longest suit. On this hand you should bid 1♥. Partner is forcing you to bid so you don't promise any points at all. Remember, partner has shown some hearts.
Responding to a takeout double.
As soon as any player makes a bid, the double is cancelled. You don't have to bid this time because East bid. East's bid cancelled the double.
East's bid cancels the double.
On this hand next you should bid 2♥. You would have bid 1♥ even with zero points so you have 9 more than you might have had. Tell partner the good news! It might seem dangerous to jump with a 4 card suit but remember that partner is promising support for the unbid suits. You'll be fine with hearts as trumps.
Jump response to a takeout double.
A double saying that you do not expect your opponents to make their contract is called a penalty double. It increases your score if the contract fails and also increases their score if the contract makes.
Doubles of notrump bids are almost always penalties. If your side has the majority of the points then doubling their 1NT contract can be more profitable than playing the hand in a contract your way.
A penalty double over 1NT showing 16+ points
Doubles of high level suit contracts tend to be for penalties.
A penalty double over 4♠