Yaku

Yaku is the Japanese Riichi term for a named, scorable winning pattern. Riichi enforces a strict rule that every winning hand must contain at least one yaku; a hand that is structurally complete (four sets and a pair) but contains no yaku is not a legal win and cannot be declared. This requirement shapes the entire game, forcing players to build toward a recognized pattern rather than just any complete shape.

Yaku range from simple to rare. Common ones include riichi itself (declaring a closed tenpai hand), tanyao (all simples, no terminals or honors), yakuhai (a triplet of dragons or the seat/round wind), and pinfu (a no-fu all-runs hand). Higher-value yaku include honitsu (half flush), chinitsu (full flush), and the closed-only iipeikou. Each yaku is worth a set number of han, and a hand's han from all its yaku are summed to determine the score, then combined with fu.

A key subtlety is that some yaku only count when the hand is fully concealed. Pinfu, riichi, menzen tsumo, and iipeikou disappear if you call a tile from another player, while others like tanyao remain valid open. Many yaku also lose one han when opened, such as honitsu and chinitsu, which is the open-hand penalty (kuisagari).

The yaku concept is specific to Japanese rules. Chinese Official (MCR), Hong Kong, and Taiwanese mahjong use fan or faan patterns that serve a similar naming role, but they generally do not impose Riichi's absolute one-yaku-minimum requirement to declare a win; in those variants a structurally complete hand can often win for a base value even with no special pattern. For example, a closed all-simples hand finished by self-draw scores tanyao plus menzen tsumo, two yaku, comfortably clearing the Riichi requirement.