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When was this rule to prolong short vowels added to Japanese?
And is there any formally accepted reason for using う and い instead of お and え? (which would seem like a more natural option, in my opinion)

There are examples where お is used after a syllable ending in ~O, are these phonetically equivalent? とお(遠) and とう(塔)?

Edit: My question, specifically is why is う used to make syllables ending in O long, when a more intuitive and natural option would have been using お (And this premise is totally subjective and my opinion, if someone disagrees, please explain)

Petruza
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1 Answers1

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Historically えい and おう were pronounced differently from ええ and おお, the first two as diphthongs and the second as long vowels. Sound changes resulted in a merger, but despite spelling reforms, the spellings remained separate.

Angelos
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    Notably, in some cases, the spelling is still indicative of the pronunciation. Take めい, for instance: generally read as //meː// in 明快{めいかい} "clear, explicit", but still as diphthong //mei// in 姪{めい} "niece". – Eiríkr Útlendi Mar 03 '17 at 20:30
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    Note here that めい for 明 is on'yomi, whereas めい for 姪 is kun'yomi. It's also notable that all instances I'm aware of where we find ええ and おお are kun'yomi: ええと, お姉{ねえ}さん, 大{おお}きい, {とお}る, etc. – Eiríkr Útlendi Mar 03 '17 at 21:07
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    This reminded me of Great Vowel Shift of English, which is similar but perhaps more drastic :) – naruto Mar 03 '17 at 22:18