|Kanji||一通 or 一気通貫|
1 han (open)
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.
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. This is particularly useful to note when needing to open the hand with such a combination. For example, having tile groups of 123, 345, 678, and 789 does not compose this yaku, despite possessing all 1-9 in the hand.
In this counter example, the tiles 1-9 for one suit are in the hand. However, ittsuu would not be counted. In fact, this tenpai hand lacks yaku on its own. As it stands, it is dependent on either chankan, haitei, or houtei. The manzu are not grouped as 123, 456, and 789. Instead, they are grouped as 123, 567, and 789. The 4-man function as the pair rather than belong to any of the sequences.
Hands aiming for ittsu often lack a particular suit, as a good number of one suit is collected in the hand.
That may only leave room for one tile group of any type, such as a triplet; and so, 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, and this disqualifies honroutou since honroutou requires either toitoi or chiitoi. Tanyao, chanta, and junchan 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.
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.
Pinfu requires a wait pattern of either ryanmen (open wait) or ryanmenten (three sided open). Any other wait pattern disqualifies pinfu. Ryanmen waits on two tile types, while ryanmenten waits on three tile types. Both of these wait patterns regarding ittsuu are subject to takame and yasume, where only one tile type produces a winning hand of higher value. Winning on that one tile would produce ittsuu; while the others do not. However, this problem is completely averted if the hand contains the full 1-9 pattern. Thus, the waiting tiles depend on the extra tile group, which renders the problem moot.
With honitsu and chinitsu
The development of honitsu and chinitsu with ittsuu is made easier, when nine tiles (1-9) out of the thirteen of the hand is already single suited. That leaves the remaining four (plus one winning tile) to be composed of the same suit or an honor tile. But once again, in the case of pinfu, takame and yakume may apply here, if applicable.
- Ikkitsuukan in Japanese Wikipedia