Question: At what point does a players’ turn begin in Mah Jongg?

Answer:  Contrary to what one might assume, a player’s turn does not begin immediately after the previous player discards a tile. This is because turns can be interrupted when another player calls (claims) a discard to make an exposure. Therefore, a player’s turn officially begins when they either pick a tile from the wall or call a discard. This ruling is not in Mah Jongg Made Easy.

Source: Back of the NMJL card, panel 1, “A player’s turn begins when they either pick or claim a tile.”