Kong

A kong (kan in Japanese, gong in Cantonese) is a meld of all four identical tiles, for example 7s-7s-7s-7s or four West winds. Despite containing four tiles, a kong still counts as a single set toward the four-sets-and-a-pair structure of a standard hand. Because only four copies of any tile exist, declaring a kong uses every one of them, and the act is announced openly even when the tiles were concealed.

There are three ways to form a kong. A closed kong (ankan) is declared from four tiles you drew yourself and is laid down face-down on the ends. An open or 'big' kong (minkan/daiminkan) claims a fourth tile from a discard when you already hold three. An added kong (shouminkan/kakan) upgrades an existing open pung by adding the fourth tile drawn later. In every case you draw a replacement tile from the dead wall, because a kong otherwise leaves your hand one tile short.

The replacement-draw rule creates real strategy. In riichi, each kong flips an extra dora indicator, which can inflate everyone's score, and winning on the replacement tile scores rinshan kaihou. A closed kong preserves a concealed hand for riichi, but an open or added kong opens you to robbing the kong (chankan), where another player may ron the very tile you add. Kongs also slightly increase the chance of an exhaustive draw since the live wall effectively shrinks.

For example, if you hold three 7s as a concealed triplet and draw the fourth 7s, you may declare ankan 7s-7s-7s-7s, draw a replacement from the dead wall, and in riichi reveal a new dora indicator before continuing play.