[gwnn]

1♥-1♠
2NT-3♥*
3♠-4♣
?

* forcing

Suppose no serious/frivolous applied here. 3♠ was natural and 4♣ was a cuebid. Does opener need to show 4♦ with any hand with a control or does he need a slam suitable hand?

[kenrexford]

The question, IMO, is whether Opener or Responder has captaincy, and whether LTTC is in play, per the agreements.

I would have Serious 3NT in play here, such that 4♦ by Opener would be LTTC, because Responder denied serious interest with the bypass of 3NT.

Lacking that, it seems that Opener is the "limited" partner, with Responder the unknown, such that the default "captain" should be Responder. Hence, 4♦ would typically show and 4♥ typically deny.

Except that this would be an incomplete analysis, as 4♠+ by Opener would logically need to be available and affecting the meaning of 4♦.

This, I think, gets to the core question, I think. Does 4♦ show a diamond control but imply overall disinterest, or does 4♦ show a second-round control (or possibly even no diamond control) but great interest?

Depends on style and agreements, I think.

[rogerclee]

4♦ should not say anything about diamonds, it just says "my hand is pretty good for slam."

[kenberg]

Seven yes votes (including mine) and no No votes. This probably explains the shortage of replies. So I'll state my reasoning in order to draw a little fire (maybe). After finding a heart fit, showing a good hand within fairly narrow limits, and showing a spade fit. I don't much see how opener can have a bad hand for slam purposes unless he has a couple of quick D losers. So he bids the D control if he has it and bids 4H if he doesn't. Since the op and the responders are heavy hitters, I suppose I am missing something with this wtp approach. I just don't see what.

Ok, Upon thinking I guess opener could have a five card heart suit headed by the 8. That might slow him down.

[awm]

I think it's essential to cuebid 4♦ with a control there. What if responder has three small diamonds and an otherwise suitable hand? How can we ever get to slam if opener bids 4♥ then?

Opener's hand is fairly limited in terms of strength here; it is not exactly clear to me what makes one opener hand more "slam suitable" than another given these limits on shape except that it would be nice to hold a lot of controls (i.e. a diamond control). And responder can always bid 4♥ over 4♦ to say: "well it was only a mild try" in any case.