Yotsugana

Different regions distinguish different sets of sounds. Using Nihon-shiki romanization:
  1 sound (zi = di = zu = du)
  2 sounds (zi = dizu = du)
  3 sounds (zi = dizudu)
  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, most dialects, such as Standard Japanese-speakers, have undergone mergers and now pronounce two sounds.