If a player discards a tile prior to picking, the tile may not be called for exposure by another player.
A player may not use a Joker for either a single tile or in a group of single tiles. This rule also applies to pairs or a group of tiles with pairs. A single tile defined is a stand-alone tile or a group of tiles that are not identical i.e. 2020 or NEWS. During the game, a player may not call to expose a group of single tiles unless the tile called is the winning tile that gives them mah jongg.