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ID83-Calling a Discard to Complete an Exposure
Question: What does the “X” mean on the card?
Answer: The letter “X” shown to a hand’s right indicates that the hand is an exposed hand, meaning you may call a tile and expose it to complete a pung, kong, quint, or sextet. A tile may be called by any player at anytime during the game. If a tile is called the game continues to the right of the player who called the tile. This sometimes causes a player to miss a turn.
Source: Mah Jongg Made Easy (2020 page 12, Choosing a Hand; pages 14-15, The Play)