What's the Difference Between Concealed and Exposed Hands?

A concealed hand is built entirely from wall draws, with no discards called. An exposed hand includes at least one group called from another player's discard and laid face-up. Some NMJL card hands require concealed play.

Concealed hands are built entirely from wall draws with no discards called. Your tiles stay hidden from opponents, which gives you more flexibility and secrecy. Exposed hands include at least one discard you called from another player, placed face-up on the table, which lets other players see part of your strategy. The NMJL card marks when a hand has to stay concealed (usually C or X). Exposing a tile on a concealed hand kills the hand. Choosing between concealed and exposed comes down to weighing the secrecy and flexibility of staying hidden against the speed and extra tile sources of calling discards.