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Singles and Pairs Must be Self-Picked Unless Ready to Win (Article ID 125)
Abstract: The 14th tile can be called for mah jongg under any circumstance. This includes waiting on a single tile or one for a pair.
Question from a player: I need to complete a pair and have one tile. If that tile is discarded, can I claim it and expose the pair?
Answer from the NMJL: “A player cannot claim a tile to expose a pair unless it is the final tile needed to complete Mah Jongg.”
Sources: Mah Jongg Made Easy 2020 page 15 para 2
Other answers from the NMJL: “You can always call your 14th tile if it results in a mah jongg.”
“You can always call your 14th tile if it results in a mah jongg including a pair.”
“A discarded tile which completes a pair cannot be called for an exposure only for mah jongg.”
“A single tile can always be called for mah jongg, whether you are playing either a concealed or an exposed hand.”
“When an exposed hand contains pairs, you can never call a discarded tile and expose a pair. A discarded tile may only be called to complete a pair if the call resulted in mah jongg”
Sources for other answers: NMJL Bulletin Q&A (1977 page 4, 1978 page 4, 1990 page 13,1989 page 5, 2009, 2011, 2012, 2013, 2018 ) https://www.nationalmahjonggleague.org