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ID42-East is Playing a Singles & Pair Hand – First Discard is a Joker
Question: When East is playing a Singles & Pair Hand, and the the first discard is a joker, it must be called. “Joker,” since there was no previously discarded tile from which to name it. This ruling is not found in Mah Jongg Made Easy.
Source: National Mah Jongg League Bulletin Q&A (1972 page 4, 1973 page 4, 1974 page 3, 1975 page 4, 1977 page 3, 1977 page 4, 1978 page 3, 1979 page 3, 1981 page 4, 1983 page 3, 1995 page 4, 2003)
https://www.nationalmahjonggleague.org