In many hanafuda games, a yaku (役, literally ‘role/position’) refers to a set of cards which, when obtained by a player, typically gives that player points for that set.
Typically there are two kinds of Yaku: Teyaku and Dekiyaku.
Teyaku are sets of cards that are formed in a player’s hand. Players typically check for them after the dealer has dealt the cards.
In some games like Koi-koi, Teyaku are refered to as “Lucky Hands”, due to them immediately stopping the round and awarding the player points. However, in games where Teyaku do not stop the round after being obtained, they are typically bad hands to play.
Dekiyaku are sets of cards that are formed within a player’s captured cards. Players consciously capture cards from the field in order to form these sets, while preventing their opponent(s) from doing the same.
In games with few to no Teyaku, the term ‘Yaku’ is used to refer to Dekiyaku.
In some games, there are also Special Yaku (特殊役, tokushuyaku), which may be sets or conditions that do not exactly match the definition of either Teyaku or Dekiyaku.
They may also refer to Teyaku or Dekiyaku which are treated differently from other normal Teyaku or Dekiyaku. For instance, in Hachi-hachi, Special Yaku are Dekiyaku that are checked separately from normal Dekiyaku, and do not stop the round unlike normal Dekiyaku.
Here will be a list of all yaku, especially those that occur in many games. I expect they will eventually be alphabetized (by English or Japanese?), with links to individual pages, which themselves can give details on any alternative names, the contents of the yaku, cultural significance of the imagery in the combination, and further links to games utilizing those yaku.
Boar-Deer-Butterfly (猪鹿蝶 [いのしかちょう, Ino–Shika–Chō])
Flower Viewing (花見酒 [はなみざけ, Hanami-Zake])
Moon Viewing (月見酒 [つきみざけ, Tsukimi-Zake])
Poetry Ribbons (Aka-Tan)
Purple Ribbons (Ao-Tan)