[Bbradley62]

What message is West's double supposed to convey?

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[aguahombre]

Maybe that it doesn't think you can make 4 spades?

[Bbradley62]

Yes, I once (when asked) explained my partner's double as "he doesn't think you can make it". But, I didn't say "he has rebiddable spades".

[barmar]

This is a forcing pass situation, and the double simply shows a defense-oriented hand. It's basically any hand that doesn't meet the criteria to bid on or make a forcing pass. There's no description for this, since it's just "everything else".

I think the descriptions of rebiddable S and 7+ 8421 points in S is coming from another rule that applies when doubling artificial bids. If 4♠ were artificial, the double would be showing various things about the suit, depending on earlier stages of the auction. However, the rule doesn't explicitly check whether the bid is artificial; it simply has a lower priority than other rules, so if we get to that rule it's assumed to be artificial. It would be pretty difficult to enumerate all the criteria that would have to be met for it to be matched explicitly; it's another type of default rule -- if we have certain holdings in the suit they bid, and no other higher priority rule matches, we double.

And as I explained in the thread about raising a weak 2 to game, when there are multiple bidding rules that match the auction, GIB combines them in the description, to avoid giving away which of the possible hand types it has. In this case, there's no description for the forcing pass case, so all you see is the description for the double of an artificial bid.

Perhaps we could improve it by adding the text "If 4S artificial" to this rule.

[Bbradley62]

or maybe just say "penalty"?