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 kanas remained even after the pairs of kanas 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 kanas, zi (ジ) and zu (ズ) with the exceptions of when there is rendaku (e.g. kannaduki) and when a second duplicate kana is voiced (e.g. tuduku, when an iteration mark may be used). An exception to this unification of spelling was permitted for regions that pronounced the four kanas 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)	[ʥi], [ʑi]		[ʣɯ], [zɯ]
North Ouu, Izumo	[ʣï]
South Ouu	[ʣɯ̈]
Kōchi	[di], [dzi]	[ʑi]	[du], [dzu]	[zu]
Kagoshima	[ʥi]	[ʑi]	[ʣu]	[zu]