Posted 2008-February-02, 18:20
Opener's 3M rebid over the double should guarantee a five-card suit 100% of the time. Otherwise, it becomes impossible to find 5-3 fits with assurance. Getting to 4-3 fits is not a good idea in an auction where you expect a bad trump break.
The way to get to your 4-4 major fits is by using choice-of-games cuebids. A cuebid of four of the opponents' suit, by either partner, is presumed to be looking for a suit and showing at least two places to play. It is not a slam try.
The auction could go P-(P), 1C-(3D), X-(P), 4D-(P), 4H-(P), 4S. Opener's 4D bid shows at least two places to play, at least one of which must be a major. Responder bids 4H, natural, in case hearts is one of those places to play. Opener then bids 4S, showing a four-card spade suit with clubs also (the only possibility for his other place to play), and the fit is found. Another way to do the same thing is P-(P), 1C-(3D), X-(P), 4C-(P), 4D-(P), 4S. Opener rebids 4C to show his main suit, and responder's 4D bid asks for a major. Responder could have one major plus club support, intending to go back to 5C if opener bids the major he doesn't have. Here, he has both majors, and doesn't care which one opener bids.
In the second auction, responder can't bid 4D and then go back to 5C to show a slam try in clubs, because that sequence simply shows that he doesn't like whichever major opener bids over 4D. The way to show a slam try in clubs, if that is what you want to do, is to bid 4NT over 4C. The partnership has already rejected notrump as a place to play (because nobody was willing to bid 3NT), so this can't be natural. It also shouldn't be any sort of Blackwood, because opener has a wide range for his 4C bid and a generalized slam invitation is more important.