We open the bidding with 1S, W doubles, and partner responds 2D. Do we want to show support for partner's minor or rebid our good 6-card major suit?
When your partner opens 1NT, you know almost exactly what kind of hand he holds. It is one of only 3 possible shapes: 4333, 4432 or 5332 - the balanced hands. And it sits in a very narrow point range: 12-14 if you are playing Acol; or 15-17 if you are playing Standard American.