Mentsu
Mentsu 「面子」 are the tile groups used for the game. The groups follow a similar pattern to the regular card game, gin rummy, where the focus is on groups of three tiles and sometimes four. They are subdivided into three types: shuntsu (sequence), koutsu (three-of-a-kind), and kantsu (four-of-a-kind). They are further categorized by their state being "open" or "closed". Forming these groups is a central part of developing a mahjong hand, because it is required for attaining tenpai and completing hands altogether. Two main factors go towards the formation of these tile groups: suit and number (or type).
The tile groups
Mahjong features four types of tiles in terms of suits: the numbered suits and honors. Every tile group must be single-suited; and thus, a tile group cannot be of mixed suit. It is improper to possess such a mix when declaring a win, as the hand would not be complete. Honor tiles take an even greater restriction, by which tile groups must be of particular tiles. Different honor tiles cannot be mixed with suited tiles or other honor tiles. Tiles among honor groups must be identical to each other. To complete these any of these tile groups, players must either self-draw from the wall or may call on discarded tiles, presuming that two out of three of a tile group are in possession. While the tile groups are subdivided into three categories, they are further subdivided into the state of being open or closed.
Shuntsu
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| Anjun - Closed | Minjun - Open |
Shuntsu 「順子」 is a tile group consisting of three sequential numbered tiles. Self-drawn sequences are called anjun 「暗順」. The open variety are minjun 「明順」.
To form a minjun, players are limited to calling tiles from kamicha, or the player from the left. Discarded tiles from the other two players, that are needed to complete this meld cannot be called upon. Furthermore, an added restriction is applied by kuikae, which applies to calls off of previously completed sequences.
Koutsu
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| Ankou - Closed | Minkou - Open |
Koutsu 「刻子」 is a tile group, consisting of three of the same type of tiles. In English, koutsu are referred as "triplets". Players may develop minkou 「明刻」 (open triplets) by calling on discarded tiles, while in possession of pairs.
Kantsu
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| Ankan - Closed | Minkan - Open | Shouminkan - Open (added) |
Tile groups involving four-of-a-kind take on a different nature than the other tile groups. Kantsu 「槓子」 must be declared by kan, to be useful. Kan is a special tile call in the game. This call forms a meld from four identical tiles. After the declaration of "kan", then a special set of rules are invoked.