|Value||2 han (Closed only)|
Chiitoitsu 「七対子」, also known as chiitoi or niconico, is one of the standard yaku hands. This hand consists of seven pairs, where no two pairs may be identical. In other words, every pair must be distinct. This hand is one of the two exceptions of the "four melds and one pair" rule of forming a hand, where the other exception is the kokushi musou. This hand is closed only.
Given the number of pairs in this hand, it is often tempting to call pon with matching discards to the pairs. However, it is generally noted that toitoi is actually a slower developing hand in this instance. This is especially the case, when the potential chiitoitsu hand sits at iishanten; and it may actually be recommended to let the hand develop into a chiitoitsu tenpai, rather than forcing toitoi. Tenpai for chiitoitsu always uses the wait of tanki (pair wait) in order to complete the seventh and last pair for the hand.
Nevertheless, transition to other yaku may actually be favorable, if the hand develops at least one ankou. In this case, players may find themselves looking to develop a toitoi hand, sanankou, or even suuankou. Whatever the case may be, this consideration does take some discretion, particularly when taking into account the number of tiles available, current point standing, and/or number of tile draws remaining.
Finally, the pairs must be distinct. A group of tiles forming ankan does not count as 2 tile pairings.
Unlike most mahjong hands, chiitoitsu is an exception to the "four groups and a pair" pattern. Instead, tiles are groups into pairs or toitsu. As such, the tile groupings cannot be interpreted other ways. For this reason, chiitoitsu is also granted a unique fu count of 25, which remains static.
The shanten count for chiitoitsu is simpler to count, which can be marked by the number of pairs in the hand.
|Tenpai||6||Obvious choice for chiitoitsu while in tenpai or 1-shanten. The choice of riichi will depend on the waiting tile, current hand, and current score.
A 1-shanten hand waits to pair one of three tile types for tenpai. Upon attaining tenpai, one of the three must be discarded.
|2||4||A hand with four pairs may consider for chiitoitsu. Only one more pair is needed to get to 1-shanten.|
|3||3||A hand with three pairs is very inefficient.|
|4||2||Attempting chiitoitsu with this start is very unreasonable. If anything a hand of this composition is better of utilizing other yaku.|
The composition of this yaku makes it very restrictive regarding its combination with other yaku. Chiitoitsu cannot be combined with any yaku dependent on tile groups of three. As the yaku demands, all tiles must be in pairs. Chiitoitsu combined with chanta gives honroutou, and it cannot be combined with junchan as there are only six different terminals, but chiitoi requires seven distinct pairs.
As for rinshan and chankan, this yaku may combine with neither. Rinshan requires a call of kan, but that is not allowed with chiitoitsu. A player may draw all four of a specific tile, but they are not treated as distinct pairs. For chankan, all chiitoitsu tenpai hands utilize tanki, or single tile wait. In order to win with a chiitoitsu hand, that single tile must be paired, and it is physically impossible for it to be used for an added kan call.
With iipeikou or ryanpeikou
A chiitoitsu hand may collect sequential series of paired tiles. The pattern would simply look like iipeikou or even ryanpeikou. For both of these yaku, neither are compatible with chiitoitsu. Once again, all tiles are considered to be pairs, instead of three-tiled-groups. For both iipeikou or ryanpeikou, sequences are required for completion, which simply does not fit the definition of chiitoitsu. This is especially the case for ryanpeikou, which actually looks like a collection of seven pairs on its own.
As a special exception to the usual scoring rules, this hand is always given 25 fu. No other fu are awarded, even though a chiitoitsu hand would qualify for 2 fu due to the pair wait which it must have. The value is not rounded up to 30 fu. This produces a unique set of scoring values for chiitoitsu hands at a given han value, although they are the same as a hand with one fewer han and 50 fu. Like all hands, once a chiitoitsu hand is valued at 5 han or more, then it is counted for mangan or greater. Chiitoitsu with mentsumo produces a 3-han hand. Since chiitoitsu can only occur in a closed hand, it is not possible to produce a 2-han chiitoitsu hand by tsumo.
|Position||2 han||3 han||4 han||5 han or more|
| Non-dealer tsumo
|Mangan, Haneman, etc.|
| Dealer tsumo
High scoring chiitoitsu hands are possible without dora. The highest possible involves riichi, ippatsu, tsumo, haitei and chinitsu. With the 2 han for chiitoitsu, the combination brings the hand to a whopping 12 han. Note that tanyao cannot be added, as the hand 22334455667788 can be scored as ryanpeikou and pinfu for an additional 2 han over chiitoitsu. In some rule variations, this hand is an optional yakuman, known as "daisharin", "daichikurin", or "daisuurin", depending on the suit..
Since all tiles in this yaku must be in pairs, then dora tiles also occur in pairs. If dora is involved, then at minimum, this yaku may possess at least 2 dora. The notable exception stems from the use of red dora, which appear as singles in their own right.
- Chiitoitsu in Japanese Wikipedia