Japanese mahjong yaku
In Japanese mahjong, yaku () is a condition that determines the value of the player's hand. It is essential to know the yaku for game strategy, since a player must have a minimum of one yaku in their hand in order to legally win a hand. Each yaku has a specific han value. Yaku conditions may be combined to produce hands of greater value. The game also features dora, that allow a hand to add han value, but they do not count as yaku. Altogether, a hand's points value increases exponentially with every han. Overview 
Yaku are somewhat similar to poker hands. They fit certain patterns based on the numbers or types of tiles included, as well as the relative value of the tiles. Unlike poker, however, multiple yaku may be combined to produce hands of greater value. Yaku are divided into three categories:
 Hands that are mandatory to be closed (menzen-nomi, 門前のみ). Hands that loses one han if the hand is open ("Eat and decrease", a literal translation of kuisagari, 喰い下がり)
 Hands that can be closed or open and retain the same han value. All hands start closed. A hand becomes open as soon as the player "calls" a tile discarded by an opponent, in order to complete a group from their own hand. This is called "melding." For example, if a player has  in their hand, and an opponent discards a , the first player may call the discard, and thus create a melded triplet. This process can also create melded sequences (e.g. 2-3-4 of the same suit), and open quads. The calling player must display the completed group by placing the tiles face-up on the table. This makes the hand "open". No open hand can become closed. The only time a player can call an opponent's discard and still have their hand remain closed is when they are calling the winning tile. For scoring purposes, the melded group is considered open, but the overall hand stays closed (menzenchin 門前清 or menzen 門前). The basic concept of a yaku is that it fits into one of three basic criteria:
 It contains a pattern of some kind
 It can consistently be formed during a game, although it does not necessarily need to be common
 It is based on specific game situations, such as discards or actions taken by other players

Finally, when it comes to points scoring, the total number of han in the hand is counted. When the han value is 4 or less, fu is also counted. The combination of the han value and fu value corresponds to a points table. List of yaku 
The following is a list of all the yaku, their names in English and Japanese, their han values, and any special conditions related to them. They are listed here in groups according to the underlying patterns that define the yaku. Example hands are given, but often, many other arrangements are possible for each yaku. All yaku can be divided into seven basic categories, depending on the dominant feature. The features are as follows: patterns based on sequences, patterns based on triplets/quads, patterns based on terminals/honors, patterns based on suits, maximum-value hands (yakuman), lucky circumstances, and special criteria. Special criteria

Yaku based on luck 
These hands are all worth one han. Yaku based on sequences

Yaku based on triplets and/or quads 
When the following hands involve triplets, quads are also acceptable. But if they require quads, triplets do not count. Each yaku is worth 2 han, regardless of whether the hand is closed or open. Yaku based on terminal or honor tiles
These hands involve terminals and/or honors, or lack thereof (such as tan'yao and yakuhai, due to their simplicity). Yaku based on suits 
The following two yaku are related to a single suit. They both lose one han when they are open. Yakuman hands 

There is a special set of hands so difficult to attain that they are automatically worth the maximum amount of points. This value, along with the hands themselves, are called yakuman (役満, or yaku-mangan 役満貫). All yakuman hands override all other han values. Some hands can form multiple yakuman (such as Big Four Winds + All Honors), but many rulesets do not award more points for this. On the other hand, some rulesets will give double the yakuman value, called daburu yakuman (ダブル役満). Additionally, regular yaku can be combined to achieve the yakuman score, a situation known as kazoe-yakuman (数え役満) ("counted yakuman"). Any hand with 13 han (or more) automatically gets the yakuman score, even if it isn't one of the following yakuman hands. The hands known as Thirteen Orphans, Four Concealed Triplets, and Big Three Dragons are considered relatively easy to complete among yakuman hands, and are collectively called "the three big families of yakuman" (Japanese: 役満御三家). Some of yakuman hands may have different names in some regions. The names used here come from the World Riichi Championship ruleset, which is also used by the American Riichi Mahjong League. Yakuman on opening hands
The following are yakuman hands completed on the first go-around. Ancient or local yaku
The following table details yaku and yakuman hands that are usually not recognized as valid but may appear in house rules. References

External links
European Mahjong Association's Riichi Ruleset

See also 
Mahjong
Japanese Mahjong scoring rules

Mahjong