Toitoi 「対々」 is a standard yaku. In English, it is referred as "all triplets"; and it also acquired the nickname of "pon palace". That's due in part with every tile group to be composed of triplets. With this in mind, this hand is immune to any requirement involving the numbered suits or honor tiles. Simply, any group of triplets would suffice. This yaku is not affected by kuisagari, where the hand is devalued in terms of han while open.
- Agari: or
Closed toi toi
- Agari: or
If this hand wins by tsumo, then it counts for the suuankou yakuman. Otherwise, it's a toi toi mixed with sanankou.
As the name implies, this yaku requires koutsu. Usually, this yaku is called open due to the ability for pon to claim discarded tiles from any player.
^ Ippatsu requires riichi to be of any use.
Toitoi with mentsumo and junchan actually form yakuman, as suuankou and chinroutou respectively. Toitoi with chanta forms honroutou.
Naturally, this yaku can be developed when the start hand, or later subsequent tile draws, produces a number of pairs or even triplets. As the yaku requires, every tile group must be a triplet, which may be acquired via the tile draws or player discards (pon). However, if the hand happens to develop with a concentration of pairs (normally 4 or 5 pairs), then a player may actually opt to develop chiitoitsu instead. A hand with 6 pairs is already tenpai for chiitoitsu, which renders a chase for toitoi to be rather pointless.
- Toitoihou in Japanese Wikipedia