A 2♣ opening shows at least 22+ points balanced, 8 1/2+ tricks in a major, or 9 1/2+ tricks in a minor.
Responses:
2♣:
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2♦: conventional, could be “waiting” with a good hand not suited to a positive response
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2♥, 2♠, 3♣, 3♦: natural and game forcing with a minimum of 8 HCP and a five-card suit with two of the top three honors
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2NT: 8+ HCP, balanced (systems off)
After a 2♦ response, opener rebids naturally:
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2♥, 2♠, 3♣, 3♦: natural, forcing to 3 of opener’s major or 4 of opener’s minor
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2NT: 22-23 points (see responses for 2NT opening above)
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3NT: 24-25 points (see responses for 3NT opening above)
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4NT: 28+ points
In response to a suit rebid, responder indicates strength.
2♣ — 2♦; 2♥:
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2♠, 3♦: natural, game force
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3♣: artificial (cheaper minor), very weak (0-4, not an ace)
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3NT: 5-7 points, balanced
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3♥: positive support, requests cue bid sequence
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4♥: fair support, discouraging slam
2♣ — 2♦; 3♦:
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3♥: "cheaper minor" (instead of 4♣)
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others: same as above