BudH, on 2019-September-27, 09:51, said:
Can someone point me to a source for either EBU or especially ACBL guidance about whether declarer is allowed to notice when either defender shows out on the line of play he describes, and using that information.
(I do seem to remember a claimer is allowed to notice an opponent showing out of a suit making a finesse a certainty through the other opponent.)
The
EBU White Book gives the following guidance: [Edit: I see that Tramticket has already posted the last paragraph above.]
EBU White Book 2019 said:
8.70.9 Claim can be seen to break down – when can claimer change line?
Suppose it is explicit or implicit in declarer’s claim that a long suit will be good (if played from the top) but in fact the suit is breaking badly and the long cards in the suit are not good. Is it normal for declarer to continue to play the suit, when the bad break has come to light, or in adjudicating the claim, can declarer be assumed to try a different suit for the tricks they need?
The L&EC is aware that different attitudes to this question are sometimes expressed, in both rulings and TD training. Some would allow claimer the benefit of noticing that the suit has broken badly (for instance) and to depart from their original line. This attitude may have been more noticeable since the new laws in 2017, although there was no real change to Law 70.
The interpretation/implementation of Law 70 in the EBU remains that it is careless, and therefore “normal”, for the claimer not to pay attention to cards played by the other side, and that claimer will continue with the original line until presumed winners are not winning.
In this case, declarer finds that the presumed
♣8 'winner' is not in fact winning when W takes
♣10 and, presumably, another, so the result on the basis of the last paragraph would be -2.