The Takeout Double is a Conventional Double. It is used in a competitive auction. It shows the equivalent of an opening hand or better and implies shortness in the suit bid by the opponents.   This type of double is usually made in the first round of the auction. It generally promises 3 card support for the unbid suits.  Vulnerability is a factor and with the right distribution a takeout double can be made with fewer than 13 points if you have the right distribution.
For example:  4/4/4/1 distribution and 11 high card points would be adequate.

Double of one major suggests 4 cards in the other major. A double of the Minor suits does not promise both majors but is expected to have 4/3 in the majors. Partner is asked to bid his best suit after a takeout double. Even with 0 points he must do something unless his RHO bids. Usually, he will just pick his longest suit -- giving preference to a major.
                 Note : Not all hands with opening values are suitable for making a takeout double.

Guidelines --Responding to a Takeout Double

A minimum response promises 0-8 points.
A JUMP in a Suit promises 9-11HCP and generally a five card suit. This is encouraging but not forcing.
If your partner has promised a major you can jump with 4 cards in that major.
A cuebid is forcing and tells your partner that your side has enough values for game.

1NT response shows a balanced hand with 6-9/(bad 10) points and a stopper in the opponent's suit.
2NTshows a balanced hand good 10-12 HCP with at least one stopper in the opponent's suit.
3NT shows a balanced hand with 13-16HCP and should promise 2 stoppers in the opponent's suit.

What if the Takeout Doubler has extra values? A free (non-competitive) raise of your suit should promise more than opening values (16-18pts). With 19-21 he would jump to 3 of your suit and with 22+ he would likely bid game.
If the Takeout Doubler rebids another suit on his next turn he is showing a big hand with 17+ HCP. Alternatively, if he bids 1NT over your minimal 1 level response he is showing a hand too strong to simply overcall 1NT --19+ pts.           All of the above is subject to partnership agreement and modifications can be made according to your style.

Partner

Hand 1

	875 7654 K76 A76

	K6 KQJ9 A43 JT32

North	East	South	West
-	1S	Dbl	Pass
2H	Pass	Pass	Pass

East opens 1S. You are sitting in the direct seat with an opening hand and you do not have a five card suit of your own to bid --
so you Double.

North knows that you have a good hand with opening values and should have 4 hearts so he bids 2H.

Since 2H is a minimal response South will pass and be content to play in a part-score.

Partner

Hand 2

	KQT64 A72 872 J4

	A986 KQ87 9 KQ64

North	East	South	West
---	1D	Double	Pass
2S	Pass	4S	Pass

East opens the bidding 1D. South has a very shapely hand and support for all the unbids. He makes a takeout double and his partner is glad to show his values by jumping to 2S.

South is pretty happy about this jump by his partner and bids 4S -- knowing his partner has 9-11 points and at least 4S -- game is pretty likely.

More References: Bridgeguys Homepage Mrbridge homepage Bridgehands Quiz on the Takeout Double