The dialects of the Japanese language fall into two primary clades, Eastern (including Tokyo) and Western (including Kyoto and Osaka), with the dialects of Kyushu and Hachijō Island often distinguished as additional branches, the latter perhaps the most divergent of all. The Ryukyuan languages of Okinawa Prefecture and the southern islands of Kagoshima Prefecture form a separate branch of the Japonic family, and are not Japanese dialects, although they are sometimes referred to as such.
History
Regional variants of Japanese have been confirmed since the Old Japanese era. The Man'yōshū, the oldest existing collection of Japanese poetry, includes poems written in dialects of the capital (Nara) and eastern Japan, but other dialects were not recorded. The recorded features of eastern dialects were rarely inherited by modern dialects, except for a few language islands such as Hachijo Island. In the Early Middle Japanese era, there were only vague records such as "rural dialects are crude". However, since the Late Middle Japanese era, features of regional dialects had been recorded in some books, for example Arte da Lingoa de Iapam, and the recorded features were fairly similar to modern dialects. The variety of Japanese dialects developed markedly during the Early Modern Japanese era (Edo period) because many feudal lords restricted the movement of people to and from other fiefs. Some isoglosses agree with old borders of han, especially in Tohoku and Kyushu. From the Nara period to the Edo period, the dialect of Kinai (now central Kansai) had been the de facto standard form of Japanese, and the dialect of Edo (now Tokyo) took over in the late Edo period.
With modernization in the late 19th century, the government and the intellectuals promoted establishment and spread of the standard language. The regional languages and dialects were slighted and suppressed, and so, locals had a sense of inferiority about their "bad" and "shameful" languages. The language of instruction was Standard Japanese, and some teachers administered punishments for using non-standard languages, particularly in the Okinawa and Tohoku regions (see also Ryukyuan languages#Modern history and Dialect card) like as vergonha in France or welsh not in Wales. From the 1940s to the 1960s, the period of Shōwa nationalism and the post-war economic miracle, the push for the replacement of regional varieties with Standard Japanese reached its peak.
Now Standard Japanese has spread throughout the nation, and traditional regional varieties are declining because of education, television, expansion of traffic, urban concentration etc. However, regional varieties have not been completely replaced with Standard Japanese. The spread of Standard Japanese means the regional varieties are now valued as "nostalgic", "heart-warming" and markers of "precious local identity", and many speakers of regional dialects have gradually overcome their sense of inferiority regarding their natural way of speaking. The contact between regional varieties and Standard Japanese creates new regional speech forms among young people, such as Okinawan Japanese.
Mutual intelligibility
In terms of mutual intelligibility, a survey in 1967 found the four most unintelligible dialects (excluding Ryūkyūan languages and Tohoku dialects) to students from Greater Tokyo are the Kiso dialect (in the deep mountains of Nagano Prefecture), the Himi dialect (in Toyama Prefecture), the Kagoshima dialect and the Maniwa dialect (in the mountains of Okayama Prefecture). The survey is based on recordings of 12- to 20- second long, of 135 to 244 phonemes, which 42 students listened and translated word-by-word. The listeners were all Keio University students who grew up in the Kanto region.
Classification
There are several generally similar approaches to classifying Japanese dialects. Misao Tōjō classified mainland Japanese dialects into three groups: Eastern, Western and Kyūshū dialects. Mitsuo Okumura classified Kyushu dialects as a subclass of Western Japanese. These theories are mainly based on grammatical differences between east and west, but Haruhiko Kindaichi classified mainland Japanese into concentric circular three groups: inside (Kansai, Shikoku, etc.), middle (Western Kantō, Chūbu, Chūgoku, etc.) and outside (Eastern Kantō, Tōhoku, Izumo, Kyushu, Hachijō, etc.) based on systems of accent, phoneme and conjugation.