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ID143-Racking vs. Tapping
Question: What is considered “racking one’s tile”?
Answer: Racking occurs when a tile is fully placed on the sloped part of ones rack. Tapping or merely touching your tile is not considered racking. Although racking one’s tile not mandatory, it is highly recommended because it closes the window of opportunity for another player to call the previously thrown tile.
Source: Mah Jongg Made Easy (2020 page 19 #11)