Doubles and redoubles

While doubles and redoubles are originally designed for penalty, they are widely used for a range of artificial purposes as they save bidding space. The table below lists the different types of doubles and redoubles in use:

Type	Range used	Meaning
Penalty (X and XX)	X: 3H or above (with exceptions) XX: 3NT or above	Looking for penalty
Takeout (X only)	Up to 3D (with exceptions)	Requests partner to bid a suit
Negative (X and XX)	Up to 2S	Shows 4 cards in the unbid majors
Support (X and XX)	Up to 2S	Shows 3-card support for an overcalled 1M response
Invitational (X and XX)	Above 2M and below 3M	Invites to game in major
Lead-directing (X only)	Unnatural bid at any level	Requests partner to lead from that suit
SOS (XX only)	Up to 3S	Requests partner to bid a new suit

To determine the nature of a double, follow the flowchart here.