Our 2C opening shows 11-15 HCP and six+ clubs, or five clubs and a four card major. Hands with strong doubletons may want to aim for 1NT by starting with 1C.
We deal with interference in the usual ways - negative doubles and support cue bids. Freebids are not forcing. The remainder of this post describes the response structure for uncontested auctions, starting with the less nuanced actions.
These are game forcing and show a six card or longer suit.
To play
This is a limit raise.
2NT is rarely needed to show an invitational notrump hand that isn’t better played in clubs or doesn’t have a four card major that needs exploration. Possibly a 3343 hand will wish to have a natural 2NT, but this shape can proceed with 2D, see below.
As a consequence we can repurpose 2NT to primarily show the 6-10 point raise, or a forcing raise. Secondary uses include showing major two suiters of invitational strength or more.
To capture the simple raise case, opener must rebid 3C which responder passes. With a forcing raise responder rebids 3D and the auction proceeds naturally.
With a 55xx hand responder rebids 3H (invitational) or 3S (forcing).
These non-forcing responses show an invitational hand (9-12ish) with a five card or longer suit. Weaker hands with club tolerance may find passing more attractive. Hands with five spades and four hearts should start with 2D (see below). Hands with four spades and five hearts can start with 2H after which opener lacking heart help will show a four card spade holding.
The remaining invitational or better hands, including those with a four card major or 54xx, and game going hands with a five card major start with 2D. The response can also be used on hands that want to invite in notrump. A carefully designed rebid structure has been created to safely capture all these cases…
2C - 2D
2H/S
Opener’s first priority is to show a four card major. If responder has the big hand with a five card major which opener didn’t hit, he can bid it now. Otherwise these auctions proceed with little drama.
2C - 2D
2NT
Opener is showing a minimum hand without a four card major (and hence six+ clubs.) Opener also has fewer than three spades, giving an invitational responder with 54xx an opportunity to play 2NT or correct to 3C. Responder’s 3D and 3H rebids show five cards and are forcing.
2C - 2D
3C
This is just like the 2NT rebid but with three spades, allowing the 54xx hand to safely bid 3S (not forcing.)
2C - 2D
3H/3S/3NT
These rebids show maximum values, six+ clubs and three cards in the suit bid, with 3NT showing three in both majors. With this specific information responder should have little problem placing the contract.
2C - 2D
3D
This shows a maximum with six+ clubs and less than three cards in each major, thus, an inability to rebid 3H, 3S or 3NT, something like 2236 or 2137.
Weak responding hands pass or bid 2NT with support, passing opener’s forced 3C rebid.
Invitational responding hands with a five+ card major start with 2 of that suit, except with 45xx start with 2D. Any invitational (or stronger) hand with a four card major or notrump interest starts with 2D. With 55xx start with 2NT, rebidding 3H. Invitational hands with club support respond 3C.
Game going responding hands, in addition to 2D, can respond 2NT with support or 55xx, or 3D, 3H or 3S with a six card suit.