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ID88-Commitment to Discarding a Tile
Question: If I discard a tile but change my mind and want to discard another tiles, can I take it back?
Answer: Once a tile has been fully named or has been placed on the table, it cannot be taken back. As the National Mah Jongg Leagues says, “Down is Down.”
Source: Mah Jongg Made Easy 2020 page 19 #7 , page 21, #18, page 23 #5