, 1
/
, 1NT
Opener has shown a balanced hand with 17-19 hcp or a balanced hand with 25-26 hcp. Opener does not deny support for responder's major, nor does he deny a 4-card or 5-card holding in the other major.
Responder may be as weak as 5+ hcp, in which case he guarantees a 5-card suit. If responder has only a 4-card suit he will have 12+ hcp.
All further bidding is natural.
Opener's 1NT rebid over responder's major suit bid is forcing.
Responder must find a bid, even if very weak. If responder rebids two of his suit, or two of a lower-ranking suit, he shows 5-7 hcp, insufficient values to force to game opposite opener's minimum of 17 hcp. A minimum opener should pass a weak rebid, or correct to two of responder's first suit. But if opener has a maximum of the 17-19 hcp range, and has some kind of a fit with responder's suit(s), he should invite game by bidding 2NT or three of responder's major suit. We must have at least 18 hcp opposite 5 hcp, so this is highly unlikely to drive us to an unmakeable contract, and it will allow responder to accept the invitation when he has 6+ hcp. When opener has 25-26 hcp, enough for game but insufficient for slam, he simply jumps to a game contract.
If responder has 8+ hcp, or a good 7 hcp, he knows that there are sufficient values for game opposite 17-19 hcp. Responder must make a reverse bid or a jump suit bid, to force the auction to game. When opener has the stronger range, 25-26 hcp, opener will invite a slam (perhaps with a natural 4NT, perhaps with a cue bid).
Finally, if responder has 12+ hcp and only a 4-card suit,
he should rebid 2NT or 3NT.
Opener will correct 3NT to 4
/
if he has 4-card support for responder's suit.
If responder bids 2NT it is forcing and asking opener to bid 4-card suits in the Baron style.
You can refer to these worked examples, Opener Balanced — Responder Bids One of a Major, to see how these bids work in practice.

,
since responder has a 5-card major and cannot sensibly prefer to play in diamonds.

(after 1
originally)
/
(after 1
originally)
originally
with 12+ hcp and 4-4 in the majors.
Opener should continue naturally and constructively, seeking to find the best contract.
.
Opener must bid 2
.
Responder can pass if he wishes to play in clubs.
If responder makes a further bid, he shows that his major suit is a good 6-card suit and game-forcing values.
If he repeats his suit he shows no singleton or void.
If he bids any other suit, including diamonds, he shows a singleton or void in that suit.
He bids 3NT (non-forcing) or 4
(forcing) to show club shortage.
Author: Chris Burton
Gravesend Bridge Club