This convention is alertable in ACBL regulations.[1]

A support double, when used, is made when RHO overcalls partner's response, which may show only 4 cards. Here is an example:

South West North East
1 / 1 1

The double shows exactly 3 s, then opener can decide whether to go on or do something else.


  1. [1] Doubles, Redoubles and Passes : Doubles, Redoubles and Passes with highly unusual or unexpected meanings

penalty double - takeout double - negative double - responsive double
support double - maximal double - Lightner double - lead-directing double
optional double