MAKING AN EXPOSURE THEN EXCHANGING A JOKER (Article 17)

Commentary: When a player claims a discard, they must already be eligible to use that tile for its intended purpose.

For example, a player cannot claim a discard to make a Pung, exchange a tile for a joker, and then add that joker to the exposure to turn the Pung into a Kong.

The same principle applies when claiming a discard for mah jongg. If the discard is needed for a pair or a grouping of singles, it must complete the hand when claimed. A player cannot claim the discard, perform a joker exchange to complete another part of the hand, and then declare mah jongg.

When claiming a discard needed for a single or pair, eligibility and timing must align.

Question from a player:  Can I call a tile for an exposure and in that same turn retrieve a joker from someone else’s exposure and add it to my own exposure? 
 
Answer from the NMJL:  You may not call a tile for an exposure and, in the same turn, retrieve a joker and add it to the exposure

Source: NMJL Bulletin 2025

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Other answers from the NMJL: 

“Usually a player may add or subtract from an exposure before they discard a tile.  However, you may not make a change to an exposure if you have exchanged a joker before discarding.  Explanation:  A player has made an exposure of 2 tiles and a joker (pung).  She has another joker in her hand.  After she exposes, she does a joker exchange.  She then wants to add the joker she already has in her hand to her exposure to make a kong.  She is not allowed to change her exposure because a new action has taken place after the exposure and before discarding.” 

Sources:  Letter from a player (2019)