Ikkitsuukan
Ikkitsuukan 「一気通貫」, or ittsuu 「一通」 for short, is a standard yaku, defined as three distinct tile groups containing 123, 456, 789 of one suit. This forms a single suit "straight" of 123456789, similar to the poker hand. This yaku can be scored open or closed, but if in an open hand, it loses 1 han of value.
| 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 - as mentioned above, 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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Ittsu requires three groups to be sequences, leaving room for only one other tile group. Yaku that require more than one triplet - sanshoku doukou, sanankou, sankantsu, shousangen, toitoi - are thus incompatible. Similarly, sanshoku and ryanpeikou are incompatible because there is no room for their patterns. Chiitoitsu, which does not use mentsu, is likewise incompatible. The 456 sequence invalidates chanta, junchan, and honroutou, while the 123 and 789 sequences invalidate tanyao.
Ittsu can often be combined with honitsu/chinitsu, as the ittsu is already composed of 9 tiles of the same suit. Pinfu also combines well; however, if the ittsu section is not complete, the hand would be subject to to takame and yasume (only one of two waiting tiles would score this yaku).
Vs 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 for both yaku because of composition, forcing players to decide one or the other.
Usage
Despite being easy to memorize, ittsu is one of the more difficult yaku to obtain.
Sanshoku is worth the same (2 han) and has a similar requirement (3 specific sequences). However, sanshoku occurs in winning hands more than twice as often as ittsu. This is likely because ittsu's 123 and 789 sequences are harder to form than other sequences.
External links
- Ikkitsuukan in Japanese Wikipedia