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ID92-Joker Exchange and Calling a Discard
Question: Can I do a Joker Exchange before or after I call a discard for an exposure and then add the Joker to my exposure?
Answer: The Joker Exchange would take place after you’ve picked up the called tile. The exposure must be able to be completed without the help of the Joker. In other words, you cannot do a Joker Exchange and add the Joker to complete your exposure. The Joker you’ve gotten in the Exchange must be placed in your rack (sloped part) and used as a part of your hand or a future Exposure. The National Mah Jongg League considers a Joker Exchange during a Call and Exposure a separate action. This rule is not round in Mah Jongg Made Easy.
Source: Letter from a player (2019)