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I noticed that there is this る coming up in 日{ひ}->昼{ひる} and 夜{よ}->夜{よる}. In addition, I haven't seen the forms with る used a lot in Classical Japanese, so the ones without る probably came first.

What is the role of this る? Does it have a meaning, or is it simply a homophone-avoiding filler like 田-んぼ and 葉-っぱ? Is it by any chance related to 雲/曇る?

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I'm not sure if anyone knows the answer, but る must be some kind of bound morpheme rather than an independent word compounded with よ/ひ, because at the time these words had formed, Japanese words never began with /r/. It does seem that よ compounded (e.g. 夜中) but よる did not. – snailboat Jul 7 '14 at 12:55
Unger writes: "Perhaps the -ru of both piru and yoru should be compared to the -ru in the adjective root akaru- 'bright' (cf. aka 'red', *aka 'brighten; open' [reconstructed in Martin 1987]), which seems to have added an intensive or transient sense. (Recall that piru could specifically denote 'noon'.) Alternatively, piru and yoru might have been reductions of longer expressions ..." – snailboat Jul 7 '14 at 12:56
Shogakukan's big dictionary makes a similar suggestion. From their entry for 昼: 「よ(夜)」に対する「よる(夜)」と同じく、「ひ(日)」に「る」のついたものという。「る」は接尾語的なものか。 Part of me wonders if the る might be the usual intransitive / passive verb suffix る, where the resultant verb form nominalized somehow; that said, the expected nominalization pattern would be り, not る. At any rate, ひる and よる as nouns were already in common use by the mid-300s to late 700s, when the Man'yōshū poems were written. Examples here. – Eiríkr Útlendi Jul 7 '14 at 17:42
"I haven't seen the forms with る used a lot" I meant relative to Modern Japanese of course. Nobody would use よ as a noun by itself now, but that seemed to be common back then. – user54609 Jul 7 '14 at 18:53
This page says -ru expresses 状態, but not sure what its sources are – dainichi Jul 8 '14 at 6:02

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