Japanese poetry is generally called waka but waka are a court poetry form that consists of three styles; chouka, tanka and sedouka – although tanka and waka are often used interchangeably. Waka were not only a pleasant pass time but also a means of communication and the basis for poetry games.
Japanese syllabary for poetry is called onji (音字, おんじ). For example, the word ‘waka’ has two onji – ‘wa’ (わ) and ‘ka’ (か). Onji are used for counting in poetry. In English, syllables are most often used as it is not always possible to use onji. For example, ‘fuyu’ (ふゆ) means winter and consists of two onji but winter while having two syllables does not have two onji.
Chouka （ちょうか）[5-7-5… 5-7-7]
Chouka are called ‘long poems’ as they have no set length. The poem can continue in the 5-7-5 until the final line of 5-7-7 is written. Chouka were normally written by one person but for a later poetry game, Renga, many people would write lines until the final line was written.
Tanka （たんか）[5-7-5 7-7]
Tanka are called ‘short poems’ as they have a set length. Tanka were originally poems of love and longing between lovers. In later years they were used to praise nature and akin it to human emotions. Tanka are written by one person.
Sedouka （せどうか）[5-7-7 5-7-7]
Sedouka is very rare and the 5-7-7 is repeated twice. Sedouka appear in the Manyoshu (万葉集, まんよしゅ), the oldest collection of Japanese poems and all but disappeared after that. Sedouka are written by one person.
Ariake: Poems of Love and Longing by the Women Courtiers of Ancient Japan
Literary History by Anders Pettersson