Responder has to choose a call before it is known what hand type opener has. In making this choice, he will be mostly concerned with catering for opener's one-bid hand types. But inevitably, his choice of call will also affect how the auction goes when opener has a two-bid hand type. For a start, it may affect the amount of bidding space available. But also, and more importantly, responder's call provides information which hopefully will be of use when opener has a two-bid hand. In order to have the most effective methods, we would really like to arrange things so that the information responder provides in trying to cater for the one-bid hands also happens to be the information that would be most useful to opener when he has a two-bid hand.
The Swedish 1C opening is a very good example of how this can be achieved. Suppose that opener's left-hand opponent overcalls 2D. Now responder will initially play opener for the dominant weak balanced hand type, which means that the methods used will be very similar to if the opening bid had been a weak 1NT. He will therefore be bidding or doubling on the following types of hands:
- Any hand strong enough for game opposite a weak balanced hand;
- Any hand with a good long suit, wanting to play a part-score in that suit;
- A hand suitable for a double. If double is take-out this means a hand with decent values, short in the opponents' suit.
He will not be bidding on mediocre hands without a long suit: these will be passed unless they are suitable for a take-out double.
This information is exactly what opener needs when he holds a two-bid hand. When opener has a strong hand like this, it is usually more important for opener to describe his hand, rather than have responder describe instead. So opener does not want responder to bid in front of him unless he has something particularly useful to say. The main thing that would be useful for opener to know about is if responder had a good suit of his own. And as we said above, this is indeed one of the hand types that responder will be bidding with. The characteristic auction for Swedish Club is something like 1C : (2D) : 2S. Here the 2S bid simply shows a hand which wants to play in 2S opposite a weak balanced hand, but the information that responder has at least five spades is also very useful when opener is strong.
This only works because the one-bid hands in the 1C opening are so well defined. In a standard system where 1C could be a weak unbalanced hand, responder would need a rather better spade suit in order to bid safely at the two-level. But in Swedish Club any half-decent five-card suit will do, such as KJxxx. If we had to pass with this sort of holding, then not only would we have difficulty in getting to a 2S part-score when that was right, but it would also be much harder to describe the hand accurately on the next round if opener turned out to have the strong option. Note also that if responder has a hand good enough to compete over an overcall, it is almost certain to be good enough for game if opener has a strong hand - this makes the continuations very easy. In summary, the weak option and the strong option of the Swedish 1C opening go particularly well together because in each case opener wants to know when responder has a moderate hand with a good suit. Of course, responder will also be bidding with strong hands of any shape, or with a hand suitable for a take-out double; again this is useful information if opener happens to have a two-bid hand. But really you see the biggest advantages of the two-way 1C opening when responder is able to show a suit.
We can contrast this with a similar-looking two-way 1C opening which shows either a minimum hand with 4+ clubs, or a strong hand of any shape. It would be very easy to build a system around this bid. And the one-bid hands here are nicely homogeneous, so responder does not have any immediate problems. However, it does not work so well, the reason being that the hands that responder will take action on have changed. In particular, he is going to be bidding on hands with moderate club support. Knowing that responder has four clubs (say) is really of very little use to opener when he has a strong hand - he is much more interested in 5-card suits, particularly major suits. And because opener's one-bid hand types no longer promise tolerance for the majors, responder will be much less willing to introduce a major suit into the auction. While we were happy to bid 1C : (2D) : 2S on a spade holding of KJxxx opposite a Swedish 1C, we would have to pass or double if the weak option just showed clubs (unless the hand was strong enough to force to the 3-level). So opener will get much less information about the majors: information is skewed towards the club suit.
The 1C opening showing "either clubs or a strong hand" is not particularly widely played. A more popular convention is a 2C opening bid which shows either a "weak two" in diamonds or a very strong hand. This has a very similar problem. If responder plays for the dominant weak hand type, then he will want to be raising diamonds on many hands with 3- or 4-card diamond support. However, if opener has a strong hand he is not interested in 3-card diamond suits at all. Indeed, a diamond "raise" may take away space that opener wanted to use to show his strong hand. Admittedly, on any given deal responder is likely to be able to guess whether opener is strong or weak. But there is much more uncertainty than you might imagine, and the price for getting it wrong is high. Furthermore, even if responder is able to work out what is going on, the partnership agreements still have to cater for both hand types, and this reduces the amount of space you have. Really, the two hand types are not particularly compatible, with information that is extremely skewed towards the diamond suit being of little use to a strong opener.
More generally, if the dominant hand type (the weak option) shows length in a particular suit, opener is going to get lots of information about responder's length in that suit, which is only going to be of use if opener's strong options have length in that suit as well. So again we see that simple natural bidding gets this right. For a standard natural 1H opening, responder is initially trying to cater for minimum hands with hearts, but if he supports the heart suit then this is still helpful to opener even when holding a much stronger hand. A more exotic example would be a 2H opening bid which shows either a "weak two" or a "strong two" in spades: here opener would be delighted to hear about spade support no matter which hand type he has (though these transfer pre-empts are problematic for other reasons, and clearly an immediate raise could make slam tries difficult with the strong type).
If you do want a wide variety of two-bid hands, then you are much better off making the weak option nebulous, or showing a balanced hand as in Swedish Club. While clearly the Swedish 1C opening has its flaws (the main one being a lack of purity in its two-bid hands, as discussed earlier), in terms of the interaction between one-bid and two-bid hands it's really as good as you can get.