Conventions in depth
After partner doubles for takeout and the opener’s side raises, your double is for takeout and asks partner to choose the suit.
Partner makes a takeout double, the opener’s side raises its suit, and you hold useful values but no obvious suit to name. Guessing which suit to bid can easily land in the wrong one. A responsive double avoids the guess.
In an auction such as (1♥) – Double – (2♥) – Double, your double is for takeout, not penalty. It shows at least two four-card unbid suits and roughly 6 or more points over a one-level raise, or 8 or more over a two-level raise. It applies through the two level; higher than that, you bid naturally.
The original doubler names the suit, choosing a level that shows the doubler’s strength:
