A double, usually at a low level, that shows willingness to bid on or to penalise the opponents but with no strong preference between the two.
Historically, this double would have been a pure penalty double. However, it is quite rare for one to have a diamond stack on this sequence. Since North might leave in the double, South needs a few diamonds for this co-operative double.
Here is a possible hand:
The best contract might be 4♠, 4♥, 3NT or 3♦ doubled. To keep all options open, South doubles.
With undisclosed length in one of the majors, North is likely to show it or, with a flattish hand and a few diamonds, pass.
