Match Point Pairs. Club Night sitting in the North seat versus "moderate" opponents.
You have no partnership agreement but you are playing with your regular partner, who clearly trusts you to interpret this double correctly. You play that 1NT, (2♥), X and 1NT, (3♥), X are take-out. Also 1NT, (2♥), P, (P), Dbl would be takeout. But this sequence is rather different.
Does partner expect you to know what the bid means once you look at your hand?
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An experienced expert partnership can afford to learn specific agreements for each such context. Ordinary players should formulate generic agreements. For example, We agree that undiscussed doubles tend to be cards or T/O. Consistently, here, the meaning of South's calls might be:
-- double = Maximal, showing sound opener with 4 card ♠ support, suggesting game in ♠s.
-- 3♠ = Minimum opener with 4 card ♠ support.
-- Pass = Other hands.
But the important thing is to have default agreements