WINDOW OF OPPORTUNITY FOR THE LAST DISCARD (Article 107)

Commentary: After a player discards, the other players have an opportunity to claim that tile. That opportunity does not remain open indefinitely. The previous discard can be claimed until the next player picks a tile from the wall and:

racks it;
names it;
discards it;
begins a joker exchange; or
declares mah jongg.

Once one of these actions occurs, the window of opportunity on the previous discard closes.

There is an exception for simultaneous action. If another player calls the last discard at the same time the next player racks a picked tile, the caller gets the discard, and the picked tile goes back into the wall.

What Does “Racks It” Mean?

Players sometimes focus on the precise moment a picked tile becomes “racked”: Is it touching the rack? Is it fully seated? Are the player’s fingers still touching it?

When a call and rack occur simultaneously, those distinctions do not matter. The simultaneous-action exception applies regardless of how far the picked tile has been placed into the rack or whether the player’s fingers are still touching it. The caller gets the discard, and the picked tile goes back into the wall.

If the tile was racked before the call, however, there is no simultaneous action to resolve. The window of opportunity on the previous discard has already closed.

The determining factor in a simultaneous call-and-rack situation is not tile position or fingers—it is timing.

Question from a player:  When is it too late to claim a discard?  If a player has not racked their picked tile, can the previous discard be called? 
 
Answer from the NMJL:  A player shall not be permitted to call a tile for an exposure or Mah Jongg: 

• After the player to the right of the discarder has drawn a tile from the wall and racked or discarded. A racked tile is defined as a tile that has been placed on the sloped part of the rack along with the players other tiles. Tapping or clicking the rack with the tile is not considered racking.
• If another player is making a Joker exchange.
• If another player has declared Mah Jongg. 

Sources: Mah Jongg Made Easy (2024 page 17 #8)

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Other answers from the NMJL:

“Once a tile is covered by the next discard it cannot be recalled for either exposure or mah jongg.” 
 
“Once a discarded tile is covered by the next discard, it cannot be called for either mah jongg or for exposure.” 
 
“Once a tile is covered by another discard, it is too late to call the previous tile.  There are no exceptions to a joker following a discard.” 
 
“Once a discarded tile is covered by another discard (even if the discard is a joker), it is too late to call the previous 3 Crack.” 
 
“Until a player has either racked or discarded a picked tile, the previous thrown tile is eligible to be called.  The drawn tile should be put back for the next player to pick.” 
 
“A tile may not be claimed for exposure or mah jongg after player next in turn has picked and racked or discarded a tile.”  
 
Sources for other answers:  National Mah Jongg League Bulletin Q&A (1976 page 3, 1977 page 4, 1978 page 4, 1979 page 3, 1981 page 5, 1998 page 5, 1990 page 5, 1995 page 5, 2000 page 6, 2002, 2003, 2007, 2008, 2010, 2011, 2014,) https://www.nationalmahjonggleague.org