If you have learnt Japanese, you most likely have been introduced to how it is spoken in Tokyo, or to a lesser extent, Osaka, Kobe, Kyoto, or any Kansai variant. However, there are some kana sounds that may or may not sound different based on the prefecture you are in. These four, づ, ず, じ, and ぢ, can sound the same, or completely different based on the prefecture. These four kana are known as the yotsugana. Historically, these characters represented four different sounds. But in Standard Japanese, based on the Tokyo dialect, some of these sounds merged to sound the same. This represents an interesting example of evolution of languages, in the context of Japanese sounds.

Back in the Heian period, in Kyoto, シ, チ, ス, ツ were pronounced as [ɕi] , [ti] , [su], and [tu] respectively. While シ and ス have conserved their pronunciations over time, チ and ツ were different. See, these two characters represented t-sounds, examples of sounds that stop all airflow called plosives. シ and ス, on the other hand, represented fricative sounds, sounds that force air through a narrow channel. The voiced equivalents, ジ, ヂ, ズ, and ヅ, were thus  [ʑi] , [di] , [zu], and [du] respectively. At the time, these sounds were clearly distinguished, thus requiring the use of separate characters to represent these sounds.

Even in 13th century Japan, however, some of the interchangeability between some sounds were apparent, and noted in dictionaries. Take the word for “whale” for example. The forms クジラ and クヂラ existed and were noted in the Kanchi’in Ruijumeigishō (観智院本『類聚名義抄』) Dictionary of 1251, showing a clear mix of some of the sounds in some regions.

Over time, by the turn of the 17th century, チ and ツ had undergone several changes, becoming affricates instead of retaining the plosive nature, by starting as a stop in airflow, then ending in a fricative. This reduced the distinction between ヂ and ヅ, which were now [ʥi] , and [ʣu] respectively, causing some mix ups between ジand ヂ and ズ and ヅ. This mix up may have led to the modern pronunciation of 水 as みず /mizu/ instead of みづ /midzu/. However, while some prefectures have adopted pronunciations rather similar to the Modern Tokyo pronunciations, islands like Kyushu still retained some, if not, all of the traditional distinctions. In anyway, by this time in Tokyo, the pronunciations of the yotsugana have become what they sound like today. While ジ ＝ [ʑi], ヂ [ʥi] ＝, ズ ＝ [zɯ],  and ヅ ＝ [dzɯ] are the general phonetic pronunciations of the yotsugana, fusions of sounds have long occurred in some prefectures at some point in history.

Today, Standard Japanese uses the fusion between じ and ぢ, and ず and づ. Yet, we see all four of these yotsugana still being written. Why is that? There were definitely consequences of fusion of such sounds in some regions, and lack thereof in other regions, on the orthography of the Japanese language. As recently as 1986, in the Gendai Kanazukai, orthography rules only permit the use of the kana じ and ず in regular use, but with notable exceptions:

• If a word undergoes rendaku from compounding, a second morpheme that would otherwise begin with the kana つ or ち in isolation would be written as づ or ぢ, like お小遣い (おこづかい, allowance).
• When つ or ち is repeated and voiced in a word, like 続く (つづく, to continue).

There were other exceptions made pertaining to word etymologies, but generally, these were the new orthography rules we are still taught today. Regional variations in interchangeability are still present, as with some dialects in the Tohoku region, or Kyushu island. It is nonetheless interesting to see how language has evolved to what it is today, yet retaining some features we would generally thinking are “archaic”.

Afterword

This post still oversimplifies other factors influencing the evolution of some sounds, like the Ryukyuan languages spoken in Okinawa, and other orthographical challenges posed over time since the 17th century. But hopefully, this post has served to introduced you to a curious aspect of Japanese phonology and orthography.