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ID149-Joker Exchange and Pick From the Wall-Window of Opportunity
Question: I understand that once you pick a tile from the wall and rack it, no one else can call the previously discarded tile. How does this apply if someone picks a tile from the wall, does not rack it but then says, “I’d like your joker.” If they don’t rack the tile first, is the previously discarded tile still up for grabs?
Answer: In the game, when you pick a tile from the wall and immediately rack it, no one else can call the previously discarded tile. However, if someone picks a tile from the wall but does not rack it and instead says, “I’d like your joker,” the rule of racking does not apply in this specific case.
It is important to note that there is an inconsistency between the rule of racking to prevent others from calling the previously discarded tile (see ID64) and the exception made for Joker Exchanges. Despite this inconsistency, the rule clearly states that if a player is in the process of doing a Joker exchange, another player cannot call a previously discarded tile, even if the player has not yet racked the tile being exchanged.
Source: Mah Jongg Made Easy edition 2020. Page 24, #17.