Different regions distinguish different sets of sounds. Using the Nihon-shiki romanization system:
  1 sound (zi = di = zu = du)
  2 sounds (zi = dizu = du)
  3 sounds (zi = dizu ≠ du)
  4 sounds (zidizudu)

Yotsugana (四つ仮名, literally "four kana") are a set of four specific kana, , , , (in the Nihon-shiki romanization system: zi, di, zu, du), used in the Japanese writing system. They historically represented four distinct voiced morae (syllables) in the Japanese language. However, Standard Japanese and the dialects of most Japanese-speakers have merged those morae down to two sounds.

Modern sound usage in various dialectsEdit

Most of the far northern dialects (Tōhoku dialects and Hokkaidō) and far southern dialects (notably Okinawan Japanese) and the Ryukyuan languages (the other Japonic languages) have also mostly merged the four sounds down to one sound. However, a few dialects, mainly around Shikoku and Kyushu in the southwest, have conserved the distinction between three or even all four sounds.

In the current Tokyo dialect, on which the modern standard language is based, as well as in the widely spoken Kansai dialect, only two sounds are distinguished, as represented in the Hepburn (ji, ji, zu, zu) and Kunrei (zi, zi, zu, zu) romanization systems.

Modern kana usageEdit

The spelling differences between the four kana were retained well into the mid-20th century, long after the merger of the different sounds that they had represented. Two distinct morae remain in most mainland dialects, such as that of Tokyo.

Shortly after the end of World War II, the discrepancy between kana usage and pronunciation was rectified as part of a general orthographic reform, the Gendai Kanazukai, or modern kana orthography. Under the new rules, only the two kana じ zi and ず zu are to be used, but two notable exceptions exist:

  1. When a word exhibits sequential voicing, or rendaku, as a result of compounding, a second morpheme that would otherwise begin with the kana つ tu or ち ti in isolation (神無月 かんなづき, kannaduki for which 月 in isolation is written つき tuki);
  2. When the kana つ tu or ち ti is repeated and voiced in a word (続く つづく, tuduku).

An exception was permitted for regions that pronounced the four kana as three or four distinct sounds. After a 1986 update to the Gendai Kanazukai, the exception was replaced with a statement that the unified spelling was to be the one primarily used but that etymologically-correct spellings were still permitted.

Modern regional variantsEdit

The following table shows some of the different realizations and mergers of the Yotsugana characters throughout Japan:

Variants di zi du zu
Tokyo (standard) [d͡ʑi] ~ [ʑi] [d͡zɯᵝ] ~ [zɯᵝ]
North Tohoku, Izumo [d͡ʑi][citation needed]
South Tohoku [d͡zɯᵝ]
Kōchi (Hata, Tosa) [di] ~ [dᶻi] [ʑi] [dɯᵝ] ~ [dᶻɯᵝ] [zɯᵝ]
Kagoshima [d͡ʑi] [ʑi] [d͡zɯᵝ] [zɯᵝ]
Okinawa [d͡ʑi]