In response to fan mail from my loyal reader, I have revisited the subject of what 4NT means. A simple agreement is for 4NT to be Blackwood (or some variety of it), no matter what the sequence is. However, if you sit down with an unfamiliar partner without discussion, he will probably assume that this is not the case.
Instead, there will be a certain range of sequences he is likely to consider to be natural, inviting slam, usually those sequences where no trumps are involved. For example, raising a no trump bid to 4NT would be invitational. In addition, sequences involving Stayman and transfers would be invitational, unless a suit is agreed. Thus 1NT - 2♣ - 2♥ - 4NT would be showing around about a 19 count with four spades, inviting partner to bid slam. Similarly 1NT - 2♦ - 2♥ - 4NT would show the same strength with five hearts (and a balanced hand). However 1NT - 2♦ - 2♥ - 3♣ - 3♥ - 4NT would be Blackwood (since now hearts have been agreed).
Other sequences involving no trumps would follow similar principles - if a suit is agreed, then 4NT is Blackwood (some people still keep it as natural if a minor suit is agreed), and otherwise it is invitational. Thus 2NT - 3♣ -3♥ - 4NT would also be invitational.
If you bid 4NT directly over a suit when no trumps have not been bid, then this would be Blackwood, agreeing the last bid suit if you play Roman Keycard Blackwood. Although even this would not be the case for many experts, who include many natural 4NT bids - after all, how else do you invite slam?? But this is into the realms of deeper agreements, which we consider later.
However, there are a couple of other sequences that would generally not be considered to the Blackwood. For example, in the last newsletter we mentioned 4♦ - 4NT which should definitely be natural. Another common sequence that few would play as Blackwood would be over a 4♠ opener from the opponents. Now 4NT is best used to show at least 5-5 in two of the other suits. It is highly unlikely that you will have a strong enough hand to ask partner for aces, but may well have two good suits and want to find the best fit.
4NT - Deeper agreements
Sequences such as 1♣ -1♥ -2♣ - 4NT are much better played as natural than as Blackwood. The reason for this is not that Blackwood is not a useful convention, but simply that there are plenty of other ways of agreeing clubs and then using Blackwood - for example, many people would play that a 3♦ bid here is a splinter (since they play 2♦ as forcing), or could use 4♣ to agree clubs. If you'd like to Blackwood but first need a way to agree a suit, then it really doesn't matter whether you have a diamond shortage to make the splinter - after all, you only wish to know how many aces partner has so you don't really care if he knows what sort of hand you have. Anybody who has wanted to invite slam with perhaps a balanced 19 count here will know how difficult it can be if 4NT would be Blackwood.
A useful rule I play with some partners is that if the auction includes fourth suit forcing in it, then a later 4NT bid is natural and invitational, unless opener has jumped to 4 of a major, or unless a suit has been agreed. Thus 1♠ - 2♣ - 2♥ - 3♦ - 4NT, 1♠ - 2♣ - 2♥ - 3♦ - 3♠ - 4NT, 1♠ - 2♣ - 2♥ - 3♦ - 3♠ - 4♣ - 4♠ - 4NT are all natural, whilst 1♠ - 2♣ - 2♥ - 3♦ - 4♠ - 4NT and 1♠ - 2♣ - 2♥ - 3♦ - 3♠ - 4♦ - 4♠ - 4NT are Blackwood (in the second one, the repeat fourth suit forcing agrees the spade suit - if you have a million minors cards, bid no trumps). A consequence of this is that 1♠ - 2♣ - 2♥ - 4NT is Blackwood for hearts, since you can go via fourth suit if you want to bid 4NT naturally.
Another rule I like is that if partner agrees your club suit and at your first opportunity the last bid was 4♠, then 4NT is used as a general slam try. For example, 1♣ - (3♠) - 4♠, or 1♣ - (2♠ ) - 3♠ - (4♠). Here it is far more likely that you would like to show slam interest, than that you will want to know how many aces partner has. (Calls in brackets denote intervention.)
Finally, what about 1♣ - 1♥ - (4♠)? This sort of sequence is also one where it is unlikely you will want to Blackwood, since you could only open at the one level, and partner has only responded at the one level. Here there are several other meanings available. The most common problem I feel here is the difference between a hand like x Kx xxx AKQ10xxx which would like to bid 5♣, and the same hand but with an extra ace, which would also like to bid 5♣ . This starts to stray into the realms of the deeper than deep agreements - if you really want to get on, you could do with getting the deep ones sorted first!