Suppose you have Axx KQ1Oxx AKJ xx opposite xx Jxx Qxx AQJxx. The standard Acol auction would be 1-2-3NT, leaving responder guessing whether to bid 4 or not, and in this case he should decide not to, since opener may only have four hearts. Now consider holding AKQxx QJxx Qx Kx. The bidding starts 1-2, what should you bid now? Partner has shown eight points or more for his 2 bid, so you definitely want to be in game, but is 3NT or 3 the right call? If partner has x Axx AKJxxx xxx he will have problems over 3, and if he bids 3NT you are going to need a favourable club lie. On the other hand, if he holds x AKxx Kxxxxx xx then 3NT is going to be a silly contract missing the heart fit.
The problem in both of these cases is that too much space has been taken up in the bidding, leaving it hard to bid to the right contract. We are going to show how many players get round this problem today, by just slightly modifying the system.
In standard Acol, a 1NT response to partner's opening bid shows a hand with no suit to bid at the one level, which is either 6-7 points with any shape (that is, not strong enough for the two level), or 8-9 points balanced. In the new system for a two level response you need 10+ points (or a nice hand with 9 points). Hands that would have bid at the two level but are no longer strong enough to do so now respond 1NT. Thus the 1NT response is now any hand with 6-9 points, no suit available at the one level This actually has little impact on the 1NT response, but an enormous intact on the rest of the system. Suppose the auction starts 1-2-2NT. Traditionally this would have shown 15-16 points with a balanced hand, stronger hands having to bid 3NT. However, if partner has already shown 10+ points, and you have 15-16, he is never going to pass you in 2NT, hence you can play the 2NT bid as 15-19 points (game-forcing), thus giving you plenty of room to investigate whether you wish to play in 3NT or 4 (or maybe 6!).
Similarly, 1-2-2 and 1-2-3 are both game-forcing. 1-2-2 is not forcing, as you will quite often want to play in 2 with a minimum misfitting hand. However, 1-2-2 is forcing, since it is rare that one would wish to pass 2 (with 10+ points you would normally wish to raise diamonds, or bid 2NT. or give false (or even true) preference to 2). This frees up the 3 bid, which can now be played as a splinter bid, that is, a singleton or void diamond with club support. (Alternatively, if you don't play Acol twos some people like to play this as a strong 5-5 hand.) Playing the 2 bid as forcing gives you a lot more room to investigate the best contract.
Going back to our initial examples: Axx KQ1Oxx AKJ xx opposite xx Jxx Qxx AQJxx. Bid 1-2-2NT-3-4-4. (The 4 bid being a slam suitable hand agreeing hearts.)
AKQxx QJxx Qx Kx opposite x Axx AKJxxx xxx, or x AKxx Kxxxxx xx. For the first hand proceed 1-2-2-3-3NT (3 being fourth suit forcing); for the second, 1-2-2-4.
Finally, notice that you now have available the sequence 1-2-3NT, with no meaning. Basically you can either agree never to use this sequence, or give it a specific meaning (since it has used up a lot of space, it should be quite specific so that partner knows how to bid on after it).