In Japanese, there are a total of 46 basic kana for both Hiragana and Katakana boards, which include:
40 combinations of vowels and consonants
1 single consonant (ん)
In addition, 25 additional kana comes from modifying some of the basic kana with diacritics called dakuten (゛) and handakuten (゜), known as the balm. However, if you look at the chart above, you will notice that there are theoretically 45 combinations of vowels and consonants, not just 40. In everyday language, the sounds wi, we, wu, yi and ye not being used, why is that?
Kana never existed
In the five kana mentioned above are not used in everyday Japanese, the three syllables wu, ye and yi never seemed to exist. While, negative wu and yi does not exist because the Japanese do not use these tense pronunciations ye is thought to have been a syllable in Japanese before the invention of the Kana writing system.
Before the modern Kana system was developed, the Japanese used the Man’yōgana notation system, in which kanji was used from Chinese to denote sounds. In man’yōgana, sound ye denoted as “江”. It is believed that yin ye and e have similar pronunciations, which has led to the use of tones e to completely replace the negative ye in the Edo period. One interesting thing to see in the Katakana table is the sound e The written “エ” is derived from sound ye “江” by Man’yōgana.
Outdated Kana sounds
Kana’s sound wi and we exists in both Hiragana and Katakana, but they are now obsolete in modern Japanese and are rarely used. The main reason for both of these kana becoming obsolete is that over time, their pronunciation begins to lose the first consonant and is gradually pronounced the same as the corresponding vowels.
Wi is written as ゐ in hiragana and ヰ in katakana. Until at least the Kamakura Period, its pronunciation wi is thought to be different from the negative i. By 1946, negative wi Officially considered obsolete, however it is still used in Japanese for foreign words or various onomatopoeic words, and is written as ウ ィ. Hiragana’s wi are sometimes still used for creative purposes such as creating band names or character names in books or games.
We is written as ゑ in hiragana and ヱ in katakana. Similar to wi, minus we is supposed to be pronounced only differently e to some point after the Kamakura period, considered obsolete in 1946 and later completely replaced by e. When to pronounce we in foreign words or onomatopoeia, we usually written as ウ ェ. Although outdated, this kana is still used for creative purposes like in Yebisu beer (ヱ ビ ス), pronounced Ebisu, but uses its original spelling in 1890.
According to: isenpai.jp
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