|English||Robbing a kan|
Chankan (搶槓) is a standard yaku. It is dependent on the usage of kan, where a player may declare ron while a player calls to upgrade a minkou to a shominkan. In other words, if a player is tenpai for a tile used for an "added kan", then the player may declare a win on that tile. In most cases you are not allowed to call ron on an ankan. The exception to this is when you are in kokushi tenpai and another player's ankan is your wait.
Alternatively, this yaku may be referred to as "robbing a kan".
As defined, chankan can only be achieved when a minkou (open triplet) is upgraded to a shouminkan. In other words, another player had called pon earlier and then drew the 4th tile, to be called kan with. If a player is tenpai for that tile, then the player may declare ron.
So, when it comes to machi (wait patterns), only three basic types can be used for chankan: penchan, kanchan, and ryanmen. Some patterns that utilize any of these three may also apply. This is simply due to the number of tiles remaining (just one), for every minkou.
In the case for kokushi musou, the hand is tenpai for kokushi with one of the 13-tile types already paired; and it is waiting for the last tile type for completion. An exception to kan calls involving ankan (closed kan) is made.
Essentially, chankan combines with all the other yaku, except for rinshan, haitei, and houtei. In the case for rinshan, the possibility to combine with chankan is outright impossible, as rinshan is defined by winning from the dead wall tile draw. In the case of haitei and houtei, a player cannot call kan, when the dead wall cannot be maintained at 14-tiles. This case occurs, at the last allowable tile draw of the hand.
Chankan may work with kokushi musou, especially when a player upgrades a minkou to a shouminkan (added kan to an open pon). Furthermore, a kokushi tenpai hand may be awarded a win if another player calls an ankan (closed kan) with that particular winning tile. Thus, ron may be declared in that instance. However, this particular rule may be subject to variation, which may or may not allow the play to occur.
- Chankan in Japanese Wikipedia