|Tile types waiting||2 sided wait|
|Tiles available||4 tiles|
|Tenhou.net example||Penchan (1-2)|
A penchan machi 「辺張待ち」 is a common type of wait pattern. This wait pattern contains the tiles 1 and 2 of the same suit, or 8 and 9 of the same suit. The pattern is limited to waiting for the completion of the shuntsu with a 3 or 7. At maximum, only four tiles are available for completion.
Penchan is often used for a 1-2 or 8-9 shape within the hand, even while the entire hand has not reached tenpai.
Because the winning tile does not complete a shuntsu which was completable on two sides, winning from a penchan awards 2 fu to the hand and does not count as pinfu. Sometimes, a winning tile can be interpreted to complete either a penchan or something else, and the higher-scoring interpretation will be used.
This hand qualifies for pinfu on both winning tiles. If a 3-pin is used, it is interpreted as completing the ryanmen 4-5 with the shuntsu 1-2-3 already in place. If it were interpreted as a 1-2 penchan winning with 3-pin and 4-5-6 already in place, the hand would gain 2 fu, but lose 1 han for not scoring pinfu.
This hand, however, will interpret the 3-pin to complete the penchan 1-2 instead of the ryanmen 4-5. It doesn't qualify for pinfu anyway because of the 1-sou ankou. This interpretation makes the hand worth 40 fu instead of 30: It has 20 base fu, 8 fu from the ankou, 2 fu for winning by tsumo, 2 fu for the penchan machi, making 32 fu in all, rounded to 40.
Penchans are generally considered to be a bad shape, or unfavorable pattern. The limit stems from the need of only one tile type for completion. This limitation renders the pattern difficult for tile development. For a 1-2 penchan, the 4 of the same suit is a useful tile, on which the 1 may be discarded. A 2-4 shape can be completed with only one tile still, but it can be turned into the ryanmen 4-5 by drawing the 5 and discarding the 2, if necessary. In short, penchan offers a limited ability to alter the wait pattern, if a need arises, especially for a case of karaten, when all four tile types are not available.
The pattern is generally weak in terms of the number of tiles needed for completion. However, the pattern may be strengthened with the use of suji. A discarded 6 tile may signal the 3 tile as safe for the other players. In the case of waiting for a 7 tile, then the 4 would be discared. For either case, players may utilize penchan to "suji bait" other players into discarding seemingly safe tiles, when in fact they're the winning tiles.
Both chanta and junchan may particularly, but not necessarily, be using penchan waits. Both these yaku rely on tiles near the ends (1,2,3) and (7,8,9). Not always but often enough, they end up using penchan waits during development and at tenpai.
- Penchan in Japanese Wikipedia