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Ask the Loebs:  What is the meaning of 4NT on the auction below?


 is there any way that 4nt can be construed as quantitative rather than key card asking since we have no suit agreement as yet?

Answer:  Thanks for your question.  

Our preference is a Jacoby 2NT response with West's hand.  West is too strong to make a splinter bid.  The most you should have for a splinter bid is 15 Support Points.  Because a splinter bid consumes so much bidding space, you want it to be very specific.

Here, West chose to respond 2 which doesn't confirm a heart fit.  East's 2NT rebid is normal playing a shape focused 2/1 Game Force style.  This brings us to the question of the meaning of West's jump to 4NT.

It isn't Roman Key Card Blackwood for hearts.  West would have bid 3 and then bid 4NT to ask for key cards for hearts.   

For most expert partnerships West's 4NT bid would be key card for responder's suit (diamonds).  The reason experts use this agreement is that responder may have a powerful hand with a self sufficient suit.  If you don't have this agreement, it may not be possible for responder to ask for key cards for their suit. 
That agreement isn't common outside of expert partnerships.  For many partnerships, this sequence would be a quantitative invitation to 6NT.   With that agreement, East would pass, declining the quantitative slam try.  
Bidding 4NT is risky if you have not set the trump suit and don't have a specific agreement on the auction.  It is wise to avoid an undiscussed 4NT which may cause confusion.  Look for an auction which is easier for your partner.  See Karen Walker's "Great Mysteries: 4NT bids" article for a detailed discussion.