Calling a Discard to Complete an Exposure (Article ID 16)
Abstract: 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.
Question from a player: If I am playing a hand with an “X” can I claim a discard to complete a block of tiles?
Answer from the NMJL: “In an exposed hand which requires pungs or kongs of Flowers, a call and exposure can be made during the play at any time.”
Source: Mah Jongg Made Easy (2020 page 12, Choosing a Hand; pages 14-15, The Play)
Other Sources: National Mah Jongg League Bulletin Q&A (1971 page 5, 1972 page 5, 1982 page 3, 1983 page 4, 1984) https://www.nationalmahjonggleague.org