Responsive double
A responsive double is used after partner makes a takeout double or simple bid. It occurs when:
LHO opens a suit;
Partner makes a takeout double or simple bid;
RHO raises at most 3♠.
A double in this situation shows at least 4-4 shape in:
the major suits if the opponents bid a minor, 18+HCP at the 2-level, 20+HCP at the 3-level;
the minor suits if the opponents bid a major, 20+HCP at the 2-level, 22+HCP at the 3-level;
denying the partner suit support.
Further the calculated bid.