Melded tiles (also known as exposed, revealed, or open) are sets that contain one discarded tile and two concealed tiles. A player may claim a discard by chowing or punging the most recently discarded tile. Afterwards they reveal the two concealed tiles that would form the chow or pung and takes the discard places it next to the two. (Three concealed tiles in case of a melded kong).
The tiles have formed a permanent bond. In other words, the tiles cannot be placed back into your hand, separated, discarded or re-arranged with any other set.
Pairs cannot be melded and can only be self-drawn.
It is considered a foul if you take the discard and place it in your hand and then meld the remaining two tiles.
Arrangement of Tiles
The melded tiles are placed face up so that all players may see it. In Japanese mahjong, World Series, and Chinese Official mahjong the claimed tile is rotated 90 degrees and placed in relation to the tiles.
Pung. 5-dot is claimed from the player's opposite seat.
Chow. 3-dot is claimed from the player's lower seat.
Pung. 1-dot is claimed from the player's upper seat.