The dialects (方言, hōgen) of the Japanese language fall into two primary clades, Eastern (including modern capital Tokyo) and Western (including old capital Kyoto), with the dialects of Kyushu and Hachijō Island often distinguished as additional branches, the latter perhaps the most divergent of all.

In these works, recorded by the Christian missionaries in Japan, they regard the true colloquial Japanese as the one used by the court nobles in Kyōto.

Nevertheless, even with the political capital being moved to Edo (i.e. Tōkyō) the status of the Kyōto dialect was not threatened immediately as it was still the cultural and economic center that dominated Japan.

This dominance waned as Edo began to assert more political and economic force and made investments in its cultural development.

The Meiji government set policies in place to spread the concept of 標準語 (hyōjun-go, "standard language").

[3] 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.

[4] Now Standard Japanese has spread throughout the nation, and traditional regional varieties are declining because of education, television, expansion of traffic, urban concentration etc.

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.

[12] The Western Japanese Kansai dialect was the prestige dialect when Kyoto was the capital, and Western forms are found in literary language as well as in honorific expressions of modern Tokyo dialect (and therefore Standard Japanese), such as adverbial ohayō gozaimasu (not *ohayaku), the humble existential verb oru, and the polite negative -masen (not *-mashinai),[12] which uses the Kyoto-style negative ending -n. Because the imperial court, which put emphasis on correct polite speech, was located in Kyoto for a long time, there was greater development of honorific speech forms in Kyoto, which were borrowed into Tokyo speech.

[14] Examples of words that originated in Kyoto and were adopted by Tokyo are yaru ("to give"), kaminari ("thunder") and asatte ("two days from today").

Kagoshima dialect is so distinctive that some have classified it as a fourth branch of Japanese, alongside Eastern, Western, and the rest of Kyushu.

The relationships between the dialects are approximated in the following cladogram:[18] Kagoshima Hichiku Hōnichi Chūgoku Umpaku Shikoku Kansai Hokuriku Tōkai–Tōsan Kantō inland Hokkaidō Tōhoku coastal Hokkaidō Hachijō West geographically separated areas seem to have been influenced by Eastern traits.

One theory argues that the Eastern type speech was spread all over Japan at the beginning and later Western characteristics developed.

The eastward spread was prevented through the geography of Japan that divides East and West that separated the cultures in each of them socio-culturally until this day.

He discovered that the newest words for snail are used in the proximity of Kyoto, the old cultural center, and older forms are found in outer areas.

[19] While it is generally accepted that languages in Western Japan are older than the Tokyo dialect, there are new studies that challenge this assumption.