Rīchi is the most common yaku used in the game, hence the name rīchi mahjong. When a player "reaches" a closed tenpai, the player has the option to declare rīchi or continue the game without making the declaration, opting for a hidden tenpai instead. Rīchi is a risk/reward play. In a sense, rīchi is an equivalent to declaring "uno", except it is not mandatory. It has similarities to raising the bet in poker: other players can "fold" instead of continuing to try to win if their chances are low, so a player may choose to not declare riichi at the cost of a lower payout. Being in a closed tenpai without calling riichi is called "damaten" or silent tenpai.
Disadvantage of rīchi
- After calling rīchi, the player's hand becomes locked and remains unchangeable afterwards. In doing so, any and all subsequent tiles are to be discarded, and only winning tiles may be claimed either by tsumo or ron.
- With the hand unchangeable, the player risks falling into another player's hand. Sometimes, the opposing hand(s) could be of very high value. The example image on the right is a potential yakuman.
- Upon declaring rīchi, a player may subject the other players to play defensively, if they choose to do so. This decreases the chances of winning the hand (assuming it could be won without declaring).
Advantage of rīchi
- Immediately, rīchi is worth an additional yaku. For hands in closed tenpai with no yaku, it is acceptable for rīchi to be the only qualifying yaku. For low-valued hands, one han effectively doubles the hand's value, and it may increase the score for limit hands as well.
- Other players defending decreases their win rates. This means the player has a higher chance of winning by tsumo, and a draw is also more likely.
- Ura-dora. Aside from the revealed dora, a player winning after calling rīchi gains access to the tile underneath the dora. This tile becomes a new dora indicator, and therefore, a player may gain additional dora if applicable.
- Chance for ippatsu, or "one shot".
Kan during rīchi
Even while rīchi, it is possible to call kan. However, this is limited to the closed kan, and the player's wait must not change. Therefore, one of the tiles in a player's kan must not be part of another meld in the hand.
- (Rīchi declared)
In this hand, a drawn cannot be called for a kan because that'd break the meld and render the hand no-ten. However, if a or is drawn a kan would be acceptable, as it would not affect any of the other melds.
- (Rīchi declared)
In this hand, declaring a kan with a drawn changes the hand's wait from to just , and thus isn't allowed by the rules. Instead, a legal kan during riichi occurs when the 4th is drawn (but not discarded), as doing so does not affect the hand's waits.
If a player has declared a kan after riichi illegally, it will be discovered if the player wins, or the round ends in a draw. The offender will then pay a chombo penalty.