Jokers Can Not be Used as a Single Tiles or in a Pair (Article ID 46)
Abstract: Under no circumstance can a Joker be used in place of a single tile or a pair.
Question from a player: Can I use a joker in a pair of as a single tile?
Answer from the NMJL: “At no time can a Joker be used for a single tile or in a pair. A Joker or Jokers may be used to replace any tile or tiles (a symbol tile is not necessary) in a Pung, Kong, Quint or Sextet.”
Source: Mah Jongg Made Easy (2020 page 23, #10, page 30 para. 6)
Other answers from the NMJL:
“Joker or jokers can never be used for a single tile, or for a pair.”
“At no time can a joker be used for a single tile, in an answer or in a pair. Joker may be used to replace any tile or tiles in a pung, kong, or quint.”
“… A joker cannot be used for the single or pair tiles. A pung is three like tiles, a kong… four like tiles. In an exposed hand you may call a discarded tile and expose the kongs as example. Jokers may be used in the kongs but [tiles] in a grouping of four singletons … can only be called to complete a sequence on a declaration of mah jongg.”
Source for other answers: National Mah Jongg League Bulletin Q&A (1975 page 3, 1986 page 5, 1990 page 13) https://www.nationalmahjonggleague.org