Menzenchin tsumohou 「門前清自摸和」, otherwise referred as menzen tsumo or even simpler tsumo 「ツモ」, is a yaku involving closed hands. Literally, a player with a closed tenpai hand may win with tsumo, as this is a yaku all on its own.
With this yaku, all closed hands qualify for menzen tsumo, even if it is the only yaku. No open hand may qualify for this yaku. The instant a player calls on a discarded tile, then the player forfeits the right to claim this yaku until the next hand, just like riichi.
Riichi vs damaten
Main articles: Riichi
While in tenpai with a closed, riichi is not a required move for any player. Instead, the player opts to utilize the tactic of damaten. In some cases, the winning tile is drawn and the hand may win.
^ Ippatsu requires riichi to be of any use.
Menzen tsumo with toitoi is actually the yakuman, suuankou. Otherwise, this yaku is compatible with all yaku except for houtei and chankan. Neither of those two yaku can win by self-draw.
This yaku is typically accompanied with riichi. Of course, the use of riichi is only advisable on the current state of the game: current point standing, other player riichi calls, potentially threatening hands, tile waits, and tile availability. At times, it may simply be advisable to utilize damaten, rather than putting one self at risk.
- Menzenchin tsumohou in Japanese Wikipedia