[pescetom]

nullve, on 2022-May-08, 17:31, said:

why can't a bid be single-suited in the familiar sense and two-suited in the possibly unfamiliar sense of c. at the same time?

Not sure what you mean by "c. at the same time".
But a single suited bid shows a single suit and no other suit, even if it implies something about one or more other suits.

nullve, on 2022-May-08, 17:31, said:

Is showing length in a suit the same as guaranteeing 3+ cards in the suit?

Then the Weak 2♠ opening described by Kantar shows length in two suits, since a hand with 6 spades, 1-3 hearts and 1-4 cards in each minor must have either 6S3y22, 6S3y(31) or 6S4m(21) shape.

4+ cards is the implicit natural length for a suit shown for the first time, including the second suit of a two-suited bid.
It may be opportune to recognise 3 cards as significant length in some circumstances, but I think both the WBF policy and normal bridge discussion take 4 cards for granted when there is no obvious reason not to do so.
As mycroft says, the WBF concept of natural may be somewhat mystic but is not hard to grasp or difficult to enforce: perhaps only a little better than "I know it when I see it", but we need the eggs

[helene_t]

Maybe the BSC definition should read:

- 2x promising 4+ cards in suit A and 3+ cards in suit B is not allowed
- 2x promising 4+ cards in suit A and 3+ cards in (suit B or suit C) is presumably allowed, since some fairly agricultural natural weak 2 styles would fit (if you never open 2S with a 3-card hearts or a 7-card spades, you will always have a 3-card minor)

So a natural weak 2♦ with the style that we don't preempt with a 3-card major would be a BSC because we always have 3+ clubs.

The problem with this is that nobody discloses such a 2♦ as "diamonds and clubs but clubs could be a 3-card suit", so they get away with it.

It is IMHO not fair to allow certain meanings if and only if they are described as negative inference. I came across this as a dubious way of explaining a Wilcosz-like 2♦ opening in a way that it would not be a BSC:

"5-card multi, unbalanced, denies a 4-card suit in the other major". This is just a style of multi 2♦ (allowed), while the equivalent
"5 cards in an unknown major and 4+ cards in an unknown minor"
is not allowed.

The TD who gave this example thought the trick wouldn't fly but who knows?

Hrothgar once proposed this meta rule:
"If a bid defined as hands belonging to set T is allowed, and S is a subset of T, then the same bid defined as hands beloning to set S is also allowed"
and I think that would be very reasonable.

But above all, why can't the rule-makers write the rules in such a way that they can't possible be misunderstood? It shouldn't be difficult. Just write what is allowed, and make it explicit that everything else is disallowed. For example:
"A 2-opening in a suit must, to be a non-BSC, satisfy at least one of these criteria:
- promising 10+ HCP
- promising 5+ cards in an unknown suit which is not the opening suit (but if it happens to contain, or promises to contain, two 5-card suits, one of them can be the opening suit)
- promising 4+ cards in a known suit
- promising either 15+ HCPs OR at least one of the above
- any meaning that is more specific than an allowed meaning is also allowed (Hrothgar's rule)