Some systems are very good at showing strength on balanced hands. For example, many Italian pairs bid balanced hands something like this:
- 12-14 HCP: open 1-of-a-suit (usually 1C)
- 15-17 HCP: open 1NT
- 18-19 HCP: open 2D (showing exactly this hand)
- 20-21 HCP: open 2NT
- 22+ HCP: open 2C
Here, the strength of a balanced hand is never more than 2HCP above the minimum promised by the opening bid, even for very strong hands. This is taking the Balanced-Hands-Show-Strength principle to extremes, and of course most systems will not make it such a high priority. But even if you do not build your entire system around showing strength on balanced hands, you should still be aware of the hands where it is most important, which are those with a strength of about 15 or 16 HCP. On weaker hands there is rarely a problem, because unless you play very light openings, those hands will not be significantly better than a minimum opening bid. And once you increase the strength to 17 or 18 HCP, you start to reach hands which are usually good enough to take a second bid (though they can still be a problem if the intervention is high enough, as with the example in the previous post). So it is the hands in the middle, about 15-16 HCP, which are most likely to cause a problem.
Most weak NT systems violate Balanced-Hands-Show-Strength on these hands. Here is the very first example from part 2:
When we play a strong NT we can open 1NT and this is a very pure one-bid hand. When we play a weak NT we have to open 1C or 1H, and now it is a nightmare hand. Supposing for the moment that LHO overcalls 2S, what is going to happen now?
Well, if 2S gets passed back to us, we will have to pass it out. This hand is nowhere near worth a second bid over a 2S overcall. So it is up to partner to do something if he wants to be in game opposite a 15-16 HCP balanced hand. The problem is, if partner does do something, and finds that you actually have a weak unbalanced hand after all, you will not always have a good fit at the three-level. And if he does bid at the three level, are you invited to bid game with a balanced 15-count, or is it merely competitive? Responder really needs to have ways to do both things, which puts a lot of strain on your system.
This assumes that partner passed at his turn. But even if he made a bid, or perhaps a negative double, there still may not be any safety in us taking a second bid. In these situations, weak NT players often use opener's double to show a balanced hand (and at least 15 HCP). This gets the strength across, but it is rather dangerous since there is no guarantee that it is right for us to compete. If we had observed the Balanced-Hands-Show-Strength principle, and described the strength of these balanced hands already, then the double could be reserved for hands which genuinely wanted to compete.
People that do play Acol or K-S, or similar systems, will accept these disadvantages and hope to gain instead on other types of hands. The weak 1NT opening itself has many advantages, being both very descriptive and also a good pre-emptive bid. It is particularly useful in first seat, and when the vulnerability is favourable, because of the pre-emptive effect. But my feeling is that in other positions it comes nowhere near making up for failing to show strength on the stronger balanced hands.
But it is possible to play a weak NT without violating the Balanced-Hands-Show-Strength principle. In Fantunes, Millennium Club and Nightmare, a 1C opening shows a better-than-minimum hand: 15+ HCP if balanced and not much less than that if unbalanced. And so opening 1C does show the strength of hands in the 15-16 HCP range very well. It turns these "strong NT" hands back into pure one-bid hands, which is what they naturally are. Also many Strong Club systems do a similar thing (though they may promise slightly more strength with the 1C bid, which puts pressure on hands with 15 HCP). If you want to play a weak NT without giving yourself huge problems on the "strong NT" type, you should play a system like this.