Take-out Double (by the Defender)
When the Opponents have opened the bidding you can make a Take-out Double (up to and including 4♦) to compete in the auction. You should have 7 losers or less and either one of the following two hands :

1. With 12-15 points : the unbid 4-card major and 0-2 cards in Opponent's suit
                                   (or 43+ in the majors if neither major has been bid)
2. With 16+ points : any shape

Partner's response priorities are :

1. bid his 4-card major

2. bid NT with Enemy suit stopper

3. bid his long minor

Responses are :

    0-9 pts = bid 4+card suit (but Pass with 0-6 pts if RH Opponent makes a bid)

10-12 pts = jump bid 4+card suit

   13+ pts = bid Game

Responsive Double (by the Advancer)

1. After an immediate raise over Partner's Take-out Double a Double by the Advancer is a Responsive Double, showing 2 unbid 4-card suits.
   A Double at the 2-level shows 6+ HCP, at the 3-level shows 9+ HCP
   1. 1♦ - (DBL) - 2♦ - (DBL) =   4-4 in the majors
   2. 1♠ - (DBL) - 2♠ - (DBL) =   4-4 in the minors

2. After an immediate raise over Partner's Overcall a Double by the Advancer is a Responsive Double, showing 2 unbid 4-card suits.
   A Double at the 2-level shows 6+ HCP, at the 3-level shows 9+ HCP
   1. 1♦ - (2♣) - 2♦ - (DBL) =   4-4 in the majors, plus 2-card support in ♣s
   2. 1♣ - (1♥) - 2♣ - (DBL) =   4 spades and 4 diamonds, plus 2-card support for ♥s suit

3. After three successive bids of different suits a Double by the 4th player is Snapdragon.   It shows moderate values, a strong 5+card 4th suit, plus 2-card support for Partner's (Overcalled) suit.

   1♣ - (1♠) - 2♥ - (DBL)   shows   >   ♠ A 5   ♥ 8 7 3   ♦ K 10 9 7 4   ♣ Q 9 6