The support double is a double, as the name suggests, which shows just 3 card support for partners bid MAJOR suit. Most partnerships just play this up to the 2 level.
Also if a double is made of partners suit then a re-double by partner also shows 3 card support.
If you are playing support doubles it means that bids of partners suit show 4 card support.
Let’s look at some examples
All of these are example of support doubles.
Auction 1
- 1♣ – P – 1♥ – 1♠
- D*
Shows 3 card heart support
Auction 2
- 1♣ – 1♦ – 1♥ – 2♦
- D*
Shows 3 card heart support
Auction 3
- 1♣ – P – 1♠ – 2♦
- D*
Shows 3 card spade support
Auction 4
- 1♣ – P – 1♠ – 2♦
- 2♠
This now shows 4 card support because you didn’t double.
Auction 5
- 1♣ – P – 1♠ – D
- RD
The RD doesn’t show extra points it just shows you have an opening hand and you’ve got 3 spades in your hand.