Shanten

Shanten is a number measuring how far a hand is from tenpai, the ready state. A hand at 1-shanten needs one useful tile to reach tenpai, 2-shanten needs two such improvements, and so on; tenpai itself is sometimes written as 0-shanten. The lower the shanten count, the closer you are to being able to win, so tracking and minimizing shanten is the core of efficient hand-building.

Shanten is counted by finding the most advanced interpretation of your 13 tiles: how many complete melds, partial melds (two tiles that could become a meld), and pairs you already have, then calculating how many tile swaps are needed to assemble four melds and a pair. Because a hand can often be read multiple ways, the true shanten is the best (lowest) over all interpretations. Special hands like seven pairs and thirteen orphans have their own shanten formulas, and a good hand evaluation takes the minimum across the standard, chiitoitsu, and kokushi readings.

In practice, players use shanten to choose discards: among candidate discards, you keep the tiles that preserve the lowest shanten and the widest acceptance (the number of tiles that advance the hand). Modern riichi theory leans heavily on 'ukeire,' counting exactly how many tiles reduce your shanten, to play efficiently. Pushing toward lower shanten quickly is balanced against hand value and safety.

For example, a hand of 123m 456m 789m 22p 5s 7s is at 1-shanten: three complete chows and a pair are done, and the 5s-7s is a partial meld (a kanchan) needing 6s. Drawing 6s makes the hand tenpai (now waiting to pair or complete the last group), so 5s-7s is the shape that defines the single remaining step to readiness.