[straube]

We're interested in overcalling a nebulous 1D (potentially void) after the opponents have opened 1C.

The Midchart defines overcalls as natural if they show 4+ in the suit (and I guess that only natural overcalls are permitted).

My take then is that we're required to have 4 diamonds if their club is natural (showing 3+) but that we can use a conventional defense (e.g. a nebulous 1D) if their opening is conventional (2+ clubs).

Is that right?

The next question is whether we would be allowed to frequently violate our agreement of having 4 diamonds (against their natural opening) when holding 3 diamonds. My understanding is that a deviation of one card from an agreement does not constitute a psyche. However, if we routinely overcalled with a 3-cd suit that seems like we have an unlawful agreement and one that the opponents obviously need to know about.

[blackshoe]

It doesn't matter whether it's a psych or not. If you frequently deviate from your agreement, to the extent your partner becomes aware of it, you have a new and different agreement. That agreement may or may not be legal. Even if it is legal, failure to disclose it as required by regulation is a violation of law.

[bluejak]

There has been a long argument on rec.games.bridge, a Newsgroup, about defences to a doubleton 1♣ opening. There is a suggestion that you may get different answers dependent on to whom you talk at ACBL headquarters.

Based on [a] my reading of the charts and [b] a written reply from Mike Flader, who writes a Laws-based column in the Bridge Bulletin and answers questions sent to <rulings@acbl.org> on behalf of the ACBL, you may use any defence to a short club opening.

But you may not use a defence showing 0+ [or even 3+] diamonds to a natural 1♣, natural being a 1♣ showing three or more clubs.