Questions: Can I leave some of my tiles standing on top of my card or standing on the flat part of my rack.

Answer: All 13 tiles must be either on the sloped part of your hand (in your rack) or in an exposure or exposures on the flat part of your rack.

A player that follows the rules could potentially declare a player’s hand dead for having too few tiles. Of course, not one wants to have to stop playing for something that can be easily corrected by keeping all 13 tiles in or on your rack.

Source: Mah Jongg Made Easy 2024, page 16, RULES AND PENALTIES, #1