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CheckbackPremere qui per la versione in Italiano Checkback is a simple convention used when responder shows a 4+ card major, and opener rebids 1NT. The rewards are two-fold. In the first place it discovers whether opener has 3-card support for responder's major or not at a low level. Secondly, it allows natural non-forcing rebids at the 2-level (except in clubs). The sequences: a) 1 After opening 1
 | 
| sequence (a) 1 2 2 2 2NT - 3 hearts, maximum  | 
| sequence (b) 1 2 2 2 2NT - 3 spades, maximum  | 
So the rules are:
After a response which shows a fit, normally the responder knows what to do. A new suit is forcing to game and searches for a 4-4 fit for an eventual slam. The system, in fact, encourages, once slam is found improbable, rapidly signing off in game.
The only sign-off is 3
,
bid with long clubs, a 4-card major only, and a weak hand that can only play in clubs.
The cheapest available bid (often 2NT) is an asking relay to determine more of opener's distribution. The responses:
| 3 | |
| 3 | |
| 2/3 NT if 4-4 minors and doubleton in the other major. | 
In the case of a misfit 3
is always to play, but repeating the major and 2NT are natural, non-forcing, and suggest a contract (with rebidding the suit discouraging 2NT). The forcing bids after the response of 2
,
are therefore:
| other major, which invites natural bidding and is normally made with a semi-balanced hand or with a 2-suiter including 4+ clubs; | |
| an new suit at the 3-level (not 3 | 
So there are two sequences where responder can, after having bid checkback and having received a misfit response, inquire further. These are:
In these cases opener rebids:
| 3 | |
| 2NT with 4-4 minors | |
| 2 | 
When the opening is 1
the response 2
to the asking bid 2
 shows 5 hearts and 3 spades, minimum or maximum.
Because of this treatment, the sequence 1
- 1
;
1NT - 2
 shows 5 spades and 3 hearts too weak for Checkback, but the sequence 1
- 1
; 2
- 2
; 2
 shows 5 spades and 3 hearts, but is forcing, and at least invitational to game. 
Note! Unlike when opening 1
, the sequence 1
 - 1
;
1NT - 2
; 2
- 2NT shows 4 spades, 3 hearts, and invitational strength. An immediate 2NT denies 3 hearts.