Cooperative double

The direct double over RHO's overcall over 1NT opening is cooperative. The other doubles are penalty.

The cooperative double is a blend between takeout and penalty doubles. This cooperative double requires not only 3~4 cards in each unbid suit like a takeout double but also 2~4 cards in the called suit to secure conversion to penalty with opener's 3+ cards. Moreover, because penalty is expected, 4-3-3-3 is allowed.

This cooperative double shows 8+ points, i.e. invitational+ values.

As takeout

Opener can jump to game with maximum.

As penalty

We can defend even if opponents have an 8-card fit.

Opener often passes to convert the double to penalty. However, opener sometimes runs. The Wbridge5 program runs if there are 5+ cards in any suit. I found this strategy too naïve, which occasionally produces disasters when opponents have a 9-card fit. I suggest the runout strategy below.

Run with 5-card major or 6-card minor

It is likely better to play our own suit contract.

Try to run with doubleton called suit

Opponents may have a 9-card fit.

3-level runouts require 5+ cards

2NT is often better if there is only 7-card fit.

If opener has no suit to bid, 2NT is also a valid runout. It is good to sit with doubleton if opponents are doubled at 3-level and there is nowhere to run. Opener may also bid 3NT if this is probably optimal, e.g. with unfavorable vulnerability.