2♣ Checkback Stayman is used in the following bidding sequences, to "check back" if opener has major suit support, saying nothing additional about the club suit. Bidding Checkback implies that the responder has five cards in his major, and may have four in the other.
1m - 1M - 1NT - 2♣
The 2♣ is Checkback Stayman. Responses are as follows:
- 2♦ : No three card support for partner's suit, no four cards in other major. Minimum hand.
- 2♥/♠ : Bidding responder's major shows three, bidding the other major shows four. Minimum hand.
- 2NT : No three card support for partner's suit, no four cards in other major. Maximum hand.
- 3♥/♠ : Bidding responder's major shows three, bidding the other major shows four. Maximum hand.
You should agree with your partner how to handle the case of holding both four of the other major and three of partner's suit. You could agree to bid up the line, or support partner's suit first. If partner doesn't like your first suit, they will invite with 2NT or bid game with 3NT. You will then correct to your other suit.
2♠ Checkback Stayman is used to find a fit in spades when the 2♣ does not promise a 4M.
1NT - 2♣, 2♥ -
- 2♠ = 4 spades, not 4 hearts, invitational
- 2N = no 4 spades, invitational
- 3N = 4 spades and game values
Read more about this topic: Stayman Convention