Culbertson system is the earliest notable natural bidding system. It has the following features:
- 4-card majors
- Strong (16-18) 1NT
- Strong two bids
- Strong jump shifts
- Game-forcing jump raises
- Strong jump overcalls
- Culbertson 4-5NT
- Culbertson asking bids
It relied on honour trick strength first (it was created before the HCP scale), but later conformed to point count.
Many of the features are now considered outdated and replaced by weak variants.
Point count Edit
In NT openings, only HCPs are counted, with the following adjustments:
- add 1 point for all 4 As,
- deduct 1 point for QJ doubleton.
In suit openings, length points are counted for the opener:
- add 1 point for every card over four in trump
- add 1 point for every card over three in side suit
with the following adjustments in suit openings:
- add 1 point for all 4 As,
- deduct 1 point for K singleton or QJ doubleton,
- deduct 1 point if the hands contains at least one unsupported Q or J in a short suit
- 1 of a suit:
- any hands with 14 points or more;
- hands with 13 points, with 4-3-3-3 hands an exception;
- 12-point hands with at least a good 5-card suit and 2 defensive tricks;
- 11-point hands with intermediate cards and 2.5 defensive tricks in additional to the above.
- 1NT: 16-18 HCPs, balanced, with stoppers in at least 3 suits and doubleton no weaker than Qx.
- 2 of a suit: Strong two bids, at most 1 trick short of game.
- 2NT: 22-24 HCPs, balanced, with stoppers in all suits.
- 3NT: 25-27 HCPs, balanced, with stoppers in all suits.
- 3 or 4 of a suit: Preemptive openings.
Suit auction Edit
See sub-article /suit auction
Raises after suit opening Edit
When raising partner's suit opening, distribution points are added:
- add the difference between the trump length and the shortest side suit;
- deduct 1 point if you lack a side suit with at least 4 cards.
- raise to 2: 6-10 points
- raise to 3: 13-16 points with at least Jxxx in trump suit, at least 10 points must be from high cards.
- raise to 4: preemptive, with at most 8 HCPs.
With 11-12 points, a 1/1 or 2/1 response should be given, intending to show support later.
After the suit has been raised, opener adds 2 points, instead of 1, for each card starting from the 5th in the trump suit.
NT responses Edit
- 1NT: 6-10 points
- 2NT: 13-15 points, balanced, stopper in all unbid suits
- 3NT: 16-18 points, balanced, stopper in all unbid suits
suit responses Edit
Add length points in additional to HCP like the opener when responding with a new suit:
- 1/1: 5+ points
- 2/1: 11+ points, slightly less with strong 5-card suit or good 6-card suit
- strong jump shift: 18+ points, when containing only 18-19, either good support in opened suit or an independent suit.
opener rebids after single raise Edit
- Pass with 14-16 points
- Invite game with 17-18 points
- Bid game with 19 points or more
Notrump auction Edit
In Culbertson system, 26 points combined were usually considered needed for a major or NT game, 29 were needed for a minor suit game although NT game was still advisable, 33 were often considered a play for 6NT, 37 for 7NT.
However, as accurate bidding has improved a lot since the Culbertson system, most player go 3NT with 25 points, or even 24 points when playing IMPs vulnerable.
Raises after NT opening Edit
When raising a NT opening, additional points are counted for length:
- 1 point for a 5-card suit headed by at least a K or QT
- 4 (5) points for a 6 (7)-card suit, which is expected to be easily established
To raise 1NT, the partner must has a balanced or semi-balanced hand, just like the opening:
- 2NT: 8-9 points
- 3NT: 10-14 points
- 4NT: 15-16 points
- 6NT: 17-18 points
- 3X - 6NT: 19-20 points
- 7NT: 21 points
It is improper to raise a NT opening containing a singleton.
- 3NT: 3-8 points
- 4NT: 9-10 (6-7) points
- 6NT: 11-12 (8-9) points
- 3X - 6NT: 13-14 (10-11) points
- 7NT: 15 (12) points
However, it is dangerous to invite slam by the basis of length points only, without high cards in the long suit.
Suit responses Edit
If the opener has 5 cards in the suit responded, 1 point is added.