White Dragon as a Zero (Article ID 104)
Abstract: The White Dragon (Soap) was chosen to represent a zero because it is a blank tile. Since blank is equivalent to nothing, it is considered to be a zero. This ruling is not found in Mah Jongg Made Easy.
Question from a player: Why is a White Dragon used as a zero?
Answer from the NMJL: “Our reasoning, White Dragon is a blank tile. Blank is nothing and nothing is zero. When used as a zero it has no matching suit. Only a joker can be used in its place.”
Source: Mah Jongg Made Easy (not included); National Mah Jongg League Bulletin Q&A (1981 page 5, 2000 page 5) https://www.nationalmahjonggleague.org