Ippatsu 「一発」 is a yaku completely dependent on riichi. By definition, ippatsu requires a riichi declaration to be in effect, for an additional 1 han. Therefore, ippatsu cannot function as a stand-alone yaku.
It is awarded if the player receives a winning tile within an uninterrupted set of opponent tile draws after the riichi declaration. The earliest possible chance to win with ippatsu comes from the shimocha's discard (player to the right). The latest possible chance to win with ippatsu is with the player's next drawn tile after the riichi declaration.
While ippatsu is generally considered to be a standard yaku, certain rulesets do not use it, in order to reduce variance and lower the value of a riichii. In general, when sitting down to play a game, ippatsu is counted unless otherwise specified or agreed on before the game.
In order for ippatsu to be counted, the hand must win before the hand in riichi discards its next tile. Furthermore, no interuption to the turn must occur. Tile calls may immediately end the chance for ippatsu any time before the riichi declarer draws the next tile. This includes any calls for kan, except for a robbed kan (chankan). In sanma, the call for kita also interrupts and thus deny ippatsu. Even if the hand wins before the next tile draw, ippatsu does not count if a tile call was made previously. Thus, players may actually use the option to make tile calls to deliberately deny a player that chance for ippatsu.
- Kan is called prior to the riichi call; and the hand wins on the next draw without interruption. Therefore, ippatsu is valid.
- This hand wins on a discard before the next draw.
In this example, the hand wins from an opponent discard before the next tile draw. However, two tile calls were made prior to the win. So, ippatsu is invalid here.
Ippatsu is unique among the yaku, for it requires riichi to even be applicable. Because of this, it is impossible for a hand to score with ippatsu alone. Ippatsu can combine with chankan, as the kan call is not considered complete if someone wins off it.
At the very minimum, a hand could be riichi with ippatsu. Unlike riichi, ippatsu is not compatible with every other yaku. Two cannot mix with ippatsu: rinshan kaihou and houtei raoyui.
Regarding rinshan kaihou, the required call for kan to invoke rinshan already cancels out ippatsu. With one single tile left in the regular wall, the last chance to call kan is allowable. This may imply the rinshan draw as the "last possible tile draw". However, that tile came from the dead wall as opposed to the regular wall, which is not the definition for the haitei.
As for houtei, a riichi declaration must be made with at least 4 tiles left in the live wall. That leaves the very last possible tile draw (and discard) belonging to the riichi declarer.
The last possible chance for riichi comes, when there are eighteen tiles left in the walls. This counts the dead wall after a tile draw. So, that leaves four tiles left for regular player draws; and barring any tile calling, every player would then have only one tile draw left. For haitei then, the fourth remaining and last tile tile draw goes to the riichi caller. Naturally, tile calls may occur beforehand to deny both the ippatsu and haitei chance.
- Ippatsu in Japanese Wikipedia