Riichi

Riichi is a declaration unique to Japanese mahjong: when your hand is closed (no claimed melds) and exactly one tile from completion (tenpai), you may announce riichi, place a 1,000-point stick into the pot, and turn your discard sideways. From that moment your hand is locked: you must discard every tile you draw unless it completes the hand or is a legal closed kan that does not change your wait. The bet is forfeited if the round ends without you winning, but recovered and added to your score if you do.

Riichi's main purpose is that it is itself a yaku, worth one han. Because a closed hand winning by ron needs a yaku to be valid, riichi is often the only thing that makes an otherwise yaku-less hand legal, which is why it is one of the most-declared calls in the game. It also unlocks bonus yaku: ippatsu (winning within one uninterrupted go-around), ura dora (extra hidden dora revealed under the dora indicators only for riichi winners), and double riichi (declaring on your very first discard).

The trade-offs are significant. Declaring announces to the table that you are tenpai, inviting opponents to defend, and it removes your flexibility to fold if the board turns dangerous, since you are forced to discard your draws. You also risk the 1,000 points if you do not win or are not tenpai at an exhaustive draw.

For example, with a closed hand of 234m 567m 234p 88s waiting on 6s or 9s, you declare riichi, lay your discard sideways, and pay 1,000. If you then draw 9s next turn untouched by any call, you win with riichi, menzen tsumo, ippatsu, and any revealed ura dora stacked on top.