The takeout double is a double of a bid made by the opening side that asks partner to bid one of the unbid suits. The double usually shows 10+ points and at least three cards in each of the unbid suits. Some basic examples:

Opener	Opponent
1	Dbl

Asks partner to bid diamonds, hearts or spades -- whichever suit he has the greatest length and strength in.

Opener	Opponent1	Responder	Opponent2
1	Pass	1	Dbl

Asks partner to bid diamonds or spades.

Opener	Opponent1	Responder	Opponent2
1	Pass	1NT	Dbl

Not for penalty; asks partner to bid clubs, hearts or spades.

Responding to a takeout double

After hearing a takeout double from partner, the following actions are available:

Pass	Converts the takeout double into a penalty double. At the one- and two-levels of bidding, Pass shows a very strong trump holding, general values, and no biddable suit. A low-level Pass should never be made on a weak hand that wants to be excused from bidding. It especially should not be made on a weak hand with a handful of little trumps. At higher levels, the Pass can be based more on points and/or flat shape.
Simple suit bid	Natural, 3+ cards (usually 4+ cards) and 0-8 points.
Jump suit bid	Natural, 4+ cards and 8+ points, forcing.
Double jump suit bid	Natural, 5+ cards, 13+ points, forcing.
Simple notrump bid	Balanced with a stopper in the opponents' suit(s) and 8-11 points.
Jump notrump bid	Balanced with a stopper in the opponents' suit(s) and 11-13 points.
3NT	Balanced with a stopper in the opponents' suit(s), signoff.
Cue-bid	10+ points, artificial and forcing. Doubler's rebids are natural.

Responding to a takeout double after interference

A freely-bid suit shows 6+ points and at least a 4-card suit. If the takeout double is redoubled, all bids are natural but jump suit bids become preemptive, not strong, showing at least 5 cards and 0-8 points.

How high to play takeout doubles is a matter of partnership agreement. Many partnerships play takeout doubles through the 3-level and optional or penalty doubles higher.