Culbertson 4-5 NT convention

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For criticism see Criticism of Culbertson 4-5 NT convention
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The Culbertson 4-5 NT convention is a slam convention in contract bridge. It was invented in 1933 by Ely Culbertson. He realized that bids of 4 and 5 NT were rarely used naturally and might therefore be more usefully made into a special slam convention. The following year, Easley Blackwood, inspired by this idea, created his own convention, which gradually displaced the Culbertson one. Culbertson himself in time accepted Blackwood into his system. Ironically, his convention survived much longer in Britain, indeed long after his death it was still standard recommended usage in the Acol system, though eventually Blackwood replaced it there too. It seems never to have been deleted as an option in the Nottingham club system.

In general, a bid of 4 NT is an instance of this convention. The exceptions are

  • a raise of partner's no-trump bid
  • an overcall of opponents' opening 4-bid
  • in Acol, an opening 4 NT bid

In the following explanations, "bid suit" means a suit naturally bid by the partnership. It thus excludes cue bids and any artificial bids the partnership may be using (Culbertson himself disapproved of these, except for cue bids in opponents' suits).

A conventional 4 NT bid shows either three aces, or two aces together with the king of a bid suit. In response,

  • 5 NT shows either two aces, or an ace and the kings of all bid suits (in the latter case of course the 4 NT bidder must have three aces)
  • with an ace of a bid suit, or the kings of all bid suits,
    • either 6 of the agreed trump suit
    • or 5 of a bid suit other than the lowest (if any); the choice will depend on judgment of whether the hand has extra values
  • with an ace or void in an unbid suit, bid 5 of that suit
  • failing any of the above, sign off in 5 of the lowest bid suit

If the 4 NT bidder rebids 5 NT, this shows the partnership has all four aces.

In the original version of the convention, a direct bid of 5 NT, without bidding 4 NT first, showed either four aces, or three aces together with the king of a bid suit. Culbertson later replaced this with his grand slam force.