A bid of 'Double' can mean 2 completely different things, depending on when it is used :

• If you have a void or singleton in the suit opened by the opposition at the 1 level, and 9 to 11 points, you should bid 'Double'. This actually gives quite a lot of information to your partner, who may respond with a genuine bid.
• If the opposition has made a bid above the 1 level, and you are sure they would not make the contract, you should bid 'Double'.

Notes

• Excuse the pun, but you need to be doubly sure the opposition will not make the contract after you have doubled, if your doubling would put them into game if they make the contract. For instance, if the opposition bids 2 Hearts and you double them, if they make the contract, they get 120 points instead of 60, and have made a game.
• If a member of the opposition is confident of making the contract on which he or she has been doubled, a bid of 'Redouble' should be made. In practice, this is quite a rare event.

Example 1

Here you have an ace, 3 jacks, a doubleton, and a singleton, giving you 10 points.

If the opposition bid 1 Spade, you have a singleton spade and an appropriate number of points for doubling the opposition, so bid 'Double'.

Example 2

Here you have an ace, a king, 3 jacks, a doubleton, and a singleton, giving you 13 points.

If you had opened one heart, your partner had responded with 2 Clubs, and subsequently the opposition had ended up in a contract of 5 Diamonds, a 'Double' would seem well justified, as the chance of you making at least 3 tricks, with perhaps 20 Honours points between you, and a 5-card diamond suit, would seem very good. In this case relatively few points can be lost by doubling, but the points to be gained if the opposition goes well down are substantial, particularly if the opposition is 'vulnerable' (see 'Scoring').

Note

A 'Double' to an opposition bid of 3 Diamonds on the same hand would seem risky, as the opposition could hold a 7-card diamond suit, and be doubled into game.