The Quantitative No Trump is the bid of 4NT that asks partner to bid 6NT with a maximum (good 16 or 17), and pass with a minimum (15 or a bad 16). You might be confused because 4NT is often Blackwood - quantitative 4NT only applies under particular circumstances. Let's look at the most common times it applies, as well as some slightly more advanced ideas.
When it applies
1. Most common application - after the opening of 1NT or 2NT
So, for example
Here partner has invited you to slam, since you have a nice 16 high card points with a quality 5 card suit, a maximum for 1NT opening, you will accept.
2. Some people apply it after any natural No Trump bid
For example
Again this can be quantitative, as it was bid immediately following a natural No Trump bid. You are expected to have say 11-14 high card points, partner is saying "if you are towards the upper range, accept my slam invite, if not, simply pass 4NT".
More advanced responses to 4NT Quantitative
If you think about it, after partner bids 4NT quantitative you have a lot of bids available other than just 6NT or Pass, you have 5♣, 5♦, 5♥, 5♠, 5NT, 6♣, etc. Some players have successfully assigned meanings to them. Possible options are
1. Natural
Accepting the invite but letting partner know about a suit. A bid of a suit on the 4 level shows a good 4 card suit, a bid on the 5 level shows a good 5 card suit. Moving back to the original example
You are telling partner that you have a quality diamond suit, and perhaps 6♦ will have better (maybe much better) chances of making than 6NT.
Rule: In general it is better to play in a trump suit rather than 6NT if you have a good quality suit and fit. Don't always give up on finding a trump fit after 4NT quantitative.
2. Responding with Aces
Some people respond to Quantitative with Aces, IF they accept. What I mean is - if they do not accept the invite, they simply pass, whereas if they accept, instead of simply just bidding 6NT, they respond with their number of Aces. Just as a little bit of insurance in case we are missing two Aces, occasionally (rarely) possible.
Using the previous example hand once again
We accept the invite but let partner know we have 2 Aces, just making sure thats enough for our 6NT contract (it very likely should be, partner should not typically be hoping you have 3 Aces).
More advanced applications of 4NT Quantitative
It can be used in lots of instances and doesn't need to always follow a natural no trump, for example.
Here partner has transferred to spades and then bid 4NT. For some partnerships this could be showing 5 spades and a quantitative 4NT. For others it is just Blackwood. My recommendation is that below the expert level to only play quantitative following a natural No Trump bid (so in this instance, just treat it as Blackwood). Otherwise there will likely be too many misunderstandings.
Advantages of using 4NT as quantitative
It is considered to be quite a necessity of modern bridge. There are times when you have a pretty good hand but don't want to gamble slam, you want to involve partner in the discussion to check how good their hand is. As an example - 1NT openings, as well as 1NT rebids, have quite a large range, partner's hand can sometimes be quite poor or quite decent. Ensuring partner does not have a poor hand can sometimes be very useful.
Disadvantages and common problems
The biggest issue people face is not knowing whether a bid is quantitative or Blackwood. I have seen this a LOT and many partnerships face it, even very experienced ones that aren't quite on the same page. There are so many possible auctions where these bids can arise, and it is difficult to discuss them all. If your partner is not a regular one, limiting to just immediately following a 1NT or 2NT opening will help avoid misunderstandings. Or otherwise discuss carefully with your partner when it should and shouldn't apply - try to have a rule that will cover all the common and even uncommon auctions.
Recommendation
Overall I do recommend using the quantitative 4NT, it is sufficiently useful and common for it to add value to your bidding system. It addresses a problem that would otherwise typically be unsolvable and you would have to guess too frequently about whether to bid slam or not, rather than to involve partner in the discussion.
My recommendation for most pairs is to only play it in tightly defined situations to avoid misunderstandings. For expert pairs, using it more frequently is possible. Feel free to contact me about this topic for further discussion in relation to your system. paul.dalley@hotmail.com
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