A yaku is one of the named winning conditions defined by the riichi ruleset, and a completed hand must contain at least one to be a legal win. The winning shape alone (four melds and a pair, or a special hand) is not enough: without a yaku, the hand cannot be declared. Each ordinary yaku carries a value in han; genuine yakuman follow a separate fixed payout.
Some yaku require a fully concealed hand, such as riichi, pinfu or menzen tsumo. Others, such as tanyao and yakuhai value triplets, remain valid with called melds without losing han. Sanshoku, ittsuu, chanta, junchan, honitsu and chinitsu instead lose one han when the hand is open. For an ordinary hand, yaku han are added together before any enabled dora, uradora and aka are counted. Dora is never a yaku by itself, so it can raise a hand's value but can never make an otherwise yakuless hand winnable.
The classic trap is muyaku, a complete hand with no yaku that cannot win regardless of its dora. A call may remove riichi or a closed-only yaku, so confirm that an open yaku remains. Under Tenhou rules, an ordinary hand with 13 or more han becomes kazoe yakuman. WRC 2025 calls the same tier yonbaiman. Genuine yakuman follow their own patterns and can stack.