Question: What are “Hot and Cold Wall?”

Answer: Playing with hot and cold walls are consider table rules. Table rules are not sanctioned by the National Mah Jongg League. Since table rules are set for this variation it is not possible to list all the rules here. “Hot Wall” and “Cold Wall” table rules are played during the last wall or remaining tiles from the initial break.

Example of a “Hot Wall”: During the “Hot Wall”, all players must account for three tiles either from the discard pile, in an exposure or in their own hand. If they cannot account for three tiles and they throw the winning tile they would have to pay out the winnings for the entire table.

Example of a “Cold Wall”: During the “Cold Wall”, all players must pick their own Mah Jongg tile. In other words, you cannot call someone else’s discard for Mah Jongg.

(See also https://mahjcon.com/mahjpedia/id53)

Source:  National Mah Jongg League Bulletin (2023)