Double is a call that is only allowed when the last action was a bid by opponents. If all other players pass afterwards, the contract is worth roughly double the value. In this case, it is known as a penalty double.

The only correct form, when spoken, is the single word double. When bidding on a bidding sheet, the correct form is a letter 'X'. Abbreviations used in literatures include X, dbl.

Because a double does not take up any bidding space, it is very commonly used for conventions, as in


penalty double - takeout double - negative double - responsive double
support double - maximal double - Lightner double - lead-directing double
optional double