Let's say you're playing a strong no-trump opening, and so you open 1NT. Now suppose LHO bids 2S. and this is passed back to you. What do you do?
Pass looks pretty clear, doesn't it?
Suppose instead that LHO had overcalled 3S, would that make any difference? Well, no, now it would be insane to do anything other than pass. What about 4S? Again pass is obvious.
Of course the reason that you pass so happily in each case is that the opening 1NT bid has already described your hand very well. If it was correct for your side to do something, then your partner should have done it already. So I call this hand a "one-bid" hand: you're not planning to take a second bid (or make a double) unless forced to do so.
Contrast this with the following hand:
You deal and open 1S. LHO overcalls 2D and it comes back to you. Now maybe your choice of bid is not completely obvious, but what is clear is that you must show hearts in some way, describing your hand as a good two-suiter. Similarly if LHO had bid 3D, you would still rebid hearts at your turn. An if LHO had bid 4d? It still must be right to bid hearts. You can probably see where I'm going with this: this hand is a perfect example of a "two-bid" hand.
Perhaps this looks like a completely trivial observation, but I've found that the difference between one-bid and two-bid hands is fundamental to just about everything to do with the theory of opening bids.
Obviously the two hands above are very "pure" examples, in that the decision as to whether to take another bid was pretty much independent of the level of the opponents' intervention. More often you will be faced with a hand which is happy to take a second bid if the opponents interfere at a low level (particularly if they bid our shortest suit), but which is not good enough to take a second bid if the intervention is sufficiently high.
Note also that whether a hand is a one-bid or a two-bid hand may depend on the bidding system. For example, if you held the first hand and were playing a weak no-trump, you would have to open it 1C (or perhaps 1H), but now you would not feel that you had really described your hand until you had rebid in no-trumps. Even so, it is often helpful to describe hands as "one-bid" or "two-bid" hands independently of the system being used. My second example is probably worth two bids no matter what system is being played. Whereas a flat 12-count is only ever going to be a one-bid hand in competition. Clearly, the important factors are the shape and the strength of the hand. The stronger or more distributional a hand is, the more likely it is to be a two-bid hand.
How should all this affect our choice of bid? I think the conclusion is fairly obvious:
- When you have a one-bid hand, you should aim to describe it as well as possible in one bid.
- When you have a two-bid hand, you should aim to describe it as well as possible in two bids.
Of course, this may be too simplistic in many cases. For instance, if you have one of those hands which may or may not be worth a second bid depending on what interference you get, then you will want to describe it quite well with the first bid, while still leaving yourself easy ways to refine the description with a rebid if the auction allows. Still, the basic principle is very important. I will refer to it as the One-Bid-Or-Two principle.
Here is a very simple example: suppose you have to choose an opening bid with the following hand, playing a natural 5-card major system:
Your choice is between 1C and 1S. Since 1S promises five cards in that suit, whereas a 1C opening only promises three, it might seem like the 1S opening is a better description. But that ignores the fact that this hand is very definitely a two-bid hand, and so you should choose the opening bid which allows you to describe the hand as accurately as possible with two bids. The way to do this is to open 1C: you will virtually always be able to rebid comfortably in spades, and this will inform partner that the club suit is longer. (If the spade suit and diamond suit were switched then it would not be so clear, since while the hand would still be a two-bid hand, it might not be so easy to give a good description with those two bids.)
Finally, is there such a thing as a three-bid hand? Well, not really. That would only make sense if the opponents interfered over our opening bid and then interfered again over our rebid. Since opponents don't tend to do that very often, it's not something which is very important to consider. Indeed, if the opponents are going to interfere then the worst case is when they reach their highest level immediately. So the important thing is whether a hand is good enough to take the second bid. Hence the "One-Bid-Or-Two" distinction.