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ID32-Pick First then Exchange Jokers
Question: When it is my turn is it okay to exchange my Joker before I pick a tile from the wall?
Answer: You must pick a tile from the wall before doing a joker exchange. If you do a Joker exchange before you pick a tile, one of your opponent’s should call your hand “dead”.
Source: Mah Jongg Made Easy 2024 page 23 #2