Why are う and い used to prolong ~O and ~E, instead of お and え? When was this rule to prolong short vowels added to Japanese? And is there any formally accepted reason for using and instead of and ? (which would seem like a more natural option, in my opinion) There are examples where is used after a syllable ending in ~O, are these phonetically equivalent? () and ()? Edit: My question, specifically is why is used to make syllables ending in O long, when a more intuitive and natural option would have been using (And this premise is totally subjective and my opinion, if someone disagrees, please explain)

Historically and were pronounced differently from and , 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.