By an unpassed hand, a two-over-one response shows 13+ points and 4+ cards or support for opener’s major. It is forcing to game.
2♥ over 1♠ shows 5+ cards.
2♣ over 1♦ shows 5+ cards and does not deny a 4-card major.
Opener responds naturally to clarify the hand.
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2NT: balanced minimum, misfit minimum (generally with stoppers in the unbid suits), or balanced maximum
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3NT: intermediate hand (15-17)
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New suit: 4+ cards (reverse implies extra strength)
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Jump shift: splinter support of previously bid suit
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Rebid of original suit: no better response