[gnasher]

pran, on 2013-January-28, 16:14, said:

Please give an example where a player has a legal reason to assume that his partner has forgotten their partnership understanding from other sources than the auction.

Sometimes you can judge from your own hand that partner has probably forgotten the system. For example, if I had AKx AKx KQ xxxxx and it went

1♣-2♦ (limit raise or better in clubs)
4♣ (keycard for clubs)

I would assume that partner had forgotten our methods. I'm not sure what I would do with this assumption, but I wouldn't feel obliged to bid according to the system.

There are other factors that you can take into account, such as an opponent's reaction to an explanation, the a priori probabilities, and how obscure or unintuitive the sequence is. In the absence of UI or CPU, there is no obligation under the Laws to follow the system (though there may be regulations, like the EBU's, that make it advisable to do so).

Also, you don't have to be certain that partner has forgotten - it's perfeclty legitimate to act on the basis of probabilities, and to take into account the expected gains and losses. With no UI or CPU, if the AI tells you that your side's interests are best served by playing for partner to have forgotten the system, you are entitled to do so.

[barmar]

Is this debate really useful?

If you don't have UI, your actions are pretty much unconstrained. You're allowed to deviate from your system, or guess that partner has done so, and you don't have to justify it (so long as what you do isn't based on a CPU). Whether the clue comes from the auction or some other source is not relevant.

If you have UI, you're not allowed to use it. This is when it becomes relevant how you might conclude that partner has forgotten the system. If the UI suggests it, you'd need AI that also makes this the only LA.

[Cascade]

pran, on 2013-January-28, 16:14, said:

Please give an example where a player has a legal reason to assume that his partner has forgotten their partnership understanding from other sources than the auction.

(I accept bluejak's assertion that also opponents' auction in some situations can bring the probability of a forgotten understanding so high that it is close to certainty.)

No reason is a legal reason. I am not obligated to justify my reason for my choice of call. An assumption does not have to be based on a reason. Law 40 protects my right to choose any call, which may be based on any assumption or reason that I am free to make, provided I do not have a concealed partnership understanding.

The bottom line is that there is no law that compels me to base my calls on partnership agreements and there is a law that permits me to choose any legal call without restriction (almost) on the reason why I choose that call.

Obviously one's own hand may lead you to an assumption that partner has forgotten. You are then free to choose any call. This reasonably frequently happens in obscure auctions where I might choose a flexible or asystemic call when I believe that partner has forgotten.

[Cascade]

jillybean, on 2013-January-28, 21:27, said:

I followed what I thought was my obligation and responded to the keycard ask. It was after the game that it was suggested that I could have passed or bid 5C as it was obvious partner could not be interested in a club slam when I held club AKJ84 and that this was bridge knowledge and had nothing to do with UI. Perhaps an example of a little knowledge being dangerous?

I am not convinced by this suggestion why can partner not have a great hand that only needs you to have good trumps and if that is the case key card is a simple way to find that information.

[Vampyr]

ArtK78, on 2013-January-29, 08:44, said:

You could have won this board by refusing to answer key card correctly, but you would have damaged partnership morale by doing so.

And gotten an adverse ruling -- not to mention a reputation as a cheat.