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ANY TILE CAN BE CALLED FOR MAH JONGG (Article 178)
Commentary: If a player has all the tiles needed but one to complete a hand, the player can declare mah jongg on any discarded tile except a joker. This rule applies whether the tile is part of a block of identical tiles or is a single tile and whether playing a concealed hand or exposable hand.
Question from a Player: When you have a hand with NEWS, can you call one of the winds for mah jongg?
Answer from the NMJL: A call may never be made for a single or pair unless it is needed to declare Mah Jongg in an exposed or a concealed hand.
Source: Mah Jongg Made Easy (2024 page 14 paragraph 1)
Other answers from the league:
Any tile may be called for mah jongg. As long as you have the other 13 tiles needed to be set, any tile thrown that results in your mah jongg may be called.
Other Sources: National Mah Jongg League Bulletin (2023)