North and East are screen mates, south and west are screen mates. East received (written down) the following explanation of the auction following 2N when on lead.
3C - Checkback
3d - denies 4 spades
4c - natural
4d - preference
On west's side of the screen none of those bids were alerted.
The ten of spades was led, to the ace. The ten is consistent with a doubleton or singleton, and west believes north shape is likely 3055, he reasons (rightly or wrongly) that given the club and diamond position on any hand where north requires two club ruffs, he will go off, provided that he must use one of his heart pitches on a spade. So north to have something like Kxx - AKQxx Axxxx. Thus west switches to a diamond, at which point north claims.
East is surprised by this, as he beleives that north has shown 4 spades and so the defence is clear, and immediately enquires what explanation was given to west about 3 clubs. It is established that 3c and 3d were not alerted and the director is called. South insists that their agreement is that there is no checkback, and produces a convention card that says they play "2-way checkback over 1NT" as evidence that this implies that they do not play checkback over 2NT. The have no system file.
How would you rule?
This post has been edited by phil_20686: 2016-November-01, 16:00