The Takeout Double has been around so long that many think of it as just a natural bid, not
a convention at all.
The purpose of the Takeout Double is to get your side into the bidding after the opposition has opened
with a suit bid. If you Double their bid it does not mean that you expect to defeat the contract, but
rather that you want your partner to name his best suit.
In the standard case the Takeout Doubler must have at least 13 points, (opening hand strength), no
more than 2 cards in the Opener's suit, and at least 3 cards in every other suit. The partner of
the Doubler must not pass, even with a weak hand. Instead he usually bids his best suit.
Doubler's partner should bid at the minimum level with fewer than 9 points, jump in a good suit with
10-11 points, and cue-bid the Opener's suit with 12 or more. The cue-bid reply forces the partnership to
game.
Each partnership must agree on when a double is for Takeout, and when it is for penalties. A workable
agreement is that doubling any suit opening bid up through 3♠ is for Takeout.
A second, (less frequent), type of Takeout Double is used when you have a hand with a good 5-card or
longer suit and more than 16 points. After an opponent opens you are too strong to simply Overcall, because
partner might well pass with 9 or 10 points. Instead you Double, then at your next turn you bid your suit.
A third, (even less frequent), type of Takeout Double is used when you have a 19-22 point balanced hand
with stoppers in the suit the opponent opens. In this case you double, then at your next turn bid Notrump.
Lesson available.