Ikkitsuukan
Ikkitsuukan 「一気通貫」, or ittsuu 「一通」 for short, is a yaku, defined as three distinct tile groups containing 123, 456, 789 of one suit. Collectively, the three groups form a complete single suit straight of 1 through 9. Naturally, in English terms, this hand can also be noted as a "straight", similar to the poker hand of the same name. This hand may be played open or closed. When open, the hand loses value of one han.
| Type | Yaku |
|---|---|
| Kanji | 一通 or 一気通貫 |
| English | Straight |
| Value |
2 han 1 han (open) |
| Speed | Fast |
| Difficulty | Easy |
Tile diagram
Winning tile: 
or 
Open
Winning tile:
Formation
While the hand shows a complete 1-9 grouping from a particular suit, it is best to view the long cluster as three separate groups of: 123, 456, and 789.
Counter example
Having all the tiles from 1-9 in one suit is not sufficient to compose this yaku - they must form as sequences of 123, 456, 789.
Waiting for: or 
In this counter example, the tiles 1-9 for one suit are in the hand, but ittsuu would not be counted. Instead, they are grouped as 123, 567, and 789, with 4-man functioning as the pair.
Discard characteristics
Hands aiming for ittsu often lack a particular suit, as a good number of one suit is collected in the hand.
Compatibility
^ Ippatsu requires riichi to be of any use.
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This yaku only leaves room for one tile group of any type, such as a triplet. Therefore, any yaku that requires more than one triplet, including sanshoku doukou, toitoi, sanankou, sankantsu, and shousangen, is deemed incompatible. Chiitoitsu is likewise incompatible as it does not use mentsu. Tanyao, chanta, junchan, and honroutou are incompatible as the 456 group contains no terminals or honours, but the 123 and 789 groups contain terminals. Finally, ittsuu is incompatible with sanshoku doujun and ryanpeikou as they each require at least three sequences in a way that are incompatible with the sequences used for ittsuu.
With sanshoku
Ittsuu requires three out of the four tile groups to be composed of sequences: 123, 456, and 789. Sanshoku also requires at least three groups of sequences. Sometimes, a hand may be 1-shanten from either yaku because of composition. There comes a point where a player may need to choose one yaku over the other, depending on the circumstances.
With pinfu
Pinfu requires a wait pattern consisting of ryanmen (open wait) or a related wait like ryanmenten (three sided open). If a hand is waiting to complete the 1-9 pattern, a ryanmen wait would be subject to takame and yasume: only one tile would complete the ittsu.
However, this problem is completely averted if the hand already contains the full 1-9 pattern, with a wait depending on the extra tile group.
With honitsu and chinitsu
Ittsuu already contains nine tiles (1-9) of a single suit, so honitsu and chinitsu naturally combine with ittsuu. To form those yaku, the remaining four tiles (plus one winning tile) have to be composed of the same suit or an honor tile.
With closed hands, this combination produces some very powerful hands. With honitsu, the hand produces mangan; while it is baiman with chinitsu.
External links
- Ikkitsuukan in Japanese Wikipedia