Question: Up to what point can I call a discarded tile?

Answer: The “window of opportunity” can be defined as the period of time in which a player can call a discarded tile. The window of opportunity closes when any of the following five things occurs:

1. A player calls a discard and places the discard and/or tiles from their rack on the top of their rack
2. A player discards a tile
3. A player racks their pick from the wall. Please note that tapping or clicking a tile on one’s rack is not considered racking
4. A player announces a joker exchange (Also see Id149)  
5. A player declares Mah Jongg on a discarded tile.

Closing the window of opportunity is each player’s responsibility and thus a reminder of why it is important to pick and rack your tile rather than hold it in your hand. 

Sources: Mah Jongg Made Easy 2024, pg. 17, #8 (a), (b), (c)