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ID49-No Joker Exchanges after Declaring Mah Jongg
Question: Is it okay to call a tile for Mah Jongg and then do a Joker exchange to use the Joker to complete the win?
Answer: Once a player calls a tile and declares Mah Jongg, the called tile must be used as the winning tile. However, there are times when a player calls a tile for an exposure and after completing the exposure, does a joker exchange which results in Mah Jongg. This win would also be considered a “self-picked” win.
Source: Mah Jongg Made Easy 2020 page 23 #8
There are some situations where a player claims a discard to make an exposure then does a joker exchange with a natural tile in their hand that would then allow them to declare mah jongg.