Yotsugana

For a more detailed treatment of this topic, see the Japanese-language Wikipedia article 四つ仮名.

Different regions recognize different phonemes. From the top:
1 phoneme (zi = di = zu = du)
2 phonemes (zi = di ≠ zu = du)
3 phonemes (zi = di ≠ zu ≠ du)
4 phonemes (zi ≠ di ≠ zu ≠ du)

Yotsugana (四つ仮名, literally "four kana") refers to the four kana ジ, ヂ, ズ, ヅ (Nihon-shiki: ji, ji, zu, zu; Kunrei: zi, di, zu, du) of the Japanese language. Traditionally four distinct phonemes, currently they are either one, two, three, or four distinct phonemes depending on dialect. In the current dialect of Tokyo, it is two phonemes as represented in Hepburn romanization.

Modern kana usage

Historically, spelling differences between the four kana remained even after the pairs of kana were pronounced indistinguishably in Tokyo dialect. Shortly after the end of World War II, however, a new rule enforced the modern kana usage, or Gendai Kanazukai. The new kanazukai unified spellings to just two kana, zi (ジ) and zu (ズ), with two classes of exceptions: when there is rendaku (e.g. 神無月　かんなづき, kannaduki, where 月 in isolation is pronounced つき duki), and when a second duplicate kana is voiced (e.g. 続く つづく, tuduku), when an iteration mark may be used. An exception to this unification of spelling was permitted for regions that pronounced the four kana as three or four phonemes. After a 1986 update to the Gendai Kanazukai, this exception was replaced with a statement that the unified spelling were to be primarily used, while still permitting etymologically correct spellings.

Modern regional dialects

Dialects	di (ヂ)	zi (ジ)	du (ヅ)	zu (ズ)
Tokyo (standard)	[d͡ʑi], [ʑi]		[d͡zu͍], [zu͍]
North Tohoku, Izumo	[d͡ʑi]
South Tohoku	[d͡zu͍]
Kōchi (Hata, Tosa)	[di] ~ [dzi]	[ʑi]	[du͍] ~ [dzu͍]	[zu͍]
Kagoshima	[d͡ʑi]	[ʑi]	[d͡zu͍]	[zu͍]