Takeout double

In the card game contract bridge, a takeout double is a low-level conventional call of "Double" over an opponent's bid as a request for partner to bid his best of the unbid suits. The most common takeout double is after an opponent's opening bid of one of a suit where the double shows a hand with opening values, support for all three unbid suits (at least three cards in each) and shortness in the suit doubled (preferably, no more than two). Normally, the partner of the doubler must bid his best suit but may pass if (a) his right hand opponent intervenes or (b) on the more rare occasions when his hand is such that he wishes to convert the takeout double to a penalty double.

Commonly a double is considered for takeout whenever one of the following conditions is met (but see balancing double below):

Most common requirements to make a takeout double are:

The most common treatment is that the fewer high card points the hand possesses, the more strict are the distribution requirements (i.e. opponent's suit(s) shorter and unbid suits longer). With stronger hands a common treatment is to double first, regardless of distribution, and then make your bid even when it differs from partner's bid:

Partnership agreement is required on the level to which Takeout doubles apply. A typical treatment is that a double up to the level of 4♥ is takeout, while doubles on higher levels are for penalties or "cooperative" (i.e. the partner is supposed to pass unless his distribution and/or strength indicates that playing a contract of their own on a higher level is a better prospect). However an alternative is to play takeout doubles even higher for several reasons:

Sometimes a “shape” double can be made on less than 12 HCP and can be made with 9+ HCP but requires 4 cards in all unbid suits (if only 1 suit bid by opponents) or 5-4 in unbid suits (if 2 suits already bid by opponents)

If the RHO passes the advancer (the doubler's partner) is forced to bid should make a descriptive bid indicating suit length, high-card strength and any stoppers:

The strength requirements above are lowered when the partner is known to be stronger (e.g. an opponents' preempt is doubled), and raised when partner can be weak (as in protective position).

One conventional variation (Bungay Black Dog Double) is that 1NT shows 6+ points and the cheapest bid in a suit is a natural "weak squeak" denying 6 points.

If the RHO bids advancer is not forced to make a bid so, if one is made, it is a free bid and different strength ranges are used although the guidance on showing the shape remains the same except for the availability of the double as a new bid:

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Wikipedia