Elongated お in the volitional verb form


In native Japanese words (that I know of at least) like 通り and 大きい, to
elongate the お-sound, another お is added when written in Hiragana (instead of
う like in Sino-Japanese words). However, it just crossed my mind that the
volitional form ending - which should certainly be natively Japanese - follows
the 漢語 pattern (e.g. やろう).

Is this a remnant of the old auxiliary verb む mentioned in the answer of the
post [Origins of the Volitional
Form](https://japanese.stackexchange.com/questions/18766/origins-of-the-
volitional-form)?




As you note, there are two modern patterns for spelling //oː// in kana:
`[CONSONANT or VOWEL KANA ending in //o//]` + お, or + う.

### For the volitional ending in う

This was originally the volitional auxiliary / suffix む, which attached to the
irrealis or 未然形【みぜんけい】 ("hasn't happened yet") form.

  * For 四段活用【よだんかつよう】 verbs, the irrealis ends in //a//, so the volitional ending was effectively //-amu//. The む ending was somewhat unstable, and it shifted to just vowel う, resulting in //-au//. This then shifted phonetically to //ɔː// (kind of like English "awww"), which we still find in the 1603 _Nippo Jisho_ ("Japanese-Portuguese Dictionary", available online [here](https://books.google.com/books?id=TFJAAQAAMAAJ)) as a distinct sound from //oː//. Between then and now, //ɔː// merged with //oː//, resulting in the modern 五段活用【ごだんかつよう】 (the fifth being this //oː// ending).
  * For 上【かみ】 and 下一段【しもいちだん】 verbs, the irrealis stem is the same as the other stems -- ending in //i// for 上【かみ】 and in //e// for 下【しも】, so the volitional ending was effectively either //-imu// or //-emu//. Due to that same instability in む, this became //-iu// or //-eu//. Due to further phonetic shifts, this became first a glide //-joː//, such as 見む (//mimu//) → 見ょう (//mjoː//) and 食べむ (//tabemu//) → 食びょう (//tabjoː//), and then the //joː// ending was re-analyzed as a suffix, and the subsumed-and-contracted vowel of the stem re-appeared, as in 見よう (//mijoː//) and 食べよう (//tabejoː//).

### For Chinese-derived long-//o// vowels in kanji readings

These are sometimes the result of an earlier manifestation of this same "//a//
+ nasal" sound shift, generally occurring at the time of borrowing. 王 is one
example, realized in Middle Chinese as something like //huaŋ//. Japanese
speakers may have heard this more as //waŋ//, but earlier Japanese didn't have
any ん sound -- so the Chinese term was imported into the Japanese sound system
as //wau//, with the earlier hiragana representation of わう. Due to the //-au//
→ //-ɔː// → //-oː// sound shift described above, this became //wɔː// and then
//oː// (since //wo// doesn't really exist as a phonemic sound in Japanese).

Alternatively, they may have been //-ip// or //-ep// in Middle Chinese. That
final //-p// became //-pu// in Japanese, and then as //p// shifted to //f//,
final //-fu// softened further to //-u//, and then we see the same //-iu// or
//-eu// to //-joː// shift as above. 協 is one example, realized in Middle
Chinese as //gep//, giving rise to the 呉音【ごおん】 reading of ぎょう.

### For native-derived long-//o// vowels

Some of these arose from the same //-iu// or //-eu// shift as above. One such
example is 今日【きょう】, reconstructed as //kepu// in the earliest form. These are
spelled with the う as the vowel lengthener.

Others arose from a related shift, where the mid-word ひ or び of 人 would shift
to ふ or ぶ, and then result in an ending of either //-uto// or //-udo//.
Examples include 素人【しろうと】 (from しろ + ひと) and 仲人【なこうど】 (from なか + びと). These
are also spelled with the う as the vowel lengthener.

Still others of these arose from the lenition (softening) of a consonant, such
as 遠【とお】 from earlier //topo//, or 頰【ほお】 from earlier //popo//. These are
spelled with the お as the vowel lengthener.

There are other patterns that also occurred, as in お父【とう】さん. An earlier form
of 父 was //toto//, and the honorific form during the Edo period was
//otossan//, from a contraction of the second //-to// with the //s// in suffix
//-san//. During the Meiji period, this //otossan// was reworked by the
Ministry of Education into //otoːsan//, which was spread via textbooks as the
new standard form for this word.

### Suggestion

If you dig around in a monolingual Japanese dictionary, you'll find that the
entry often starts with a hiragana spelling that differs from the expected
modern spelling. See [the 大きい entry at
Weblio](https://www.weblio.jp/content/%E5%A4%A7%E3%81%8D%E3%81%84), for
instance, which starts with:

> おおき・い おほきい ［3］ 【大きい】

That second kana string is the old, pre-reform kana spelling, and those old
spellings give you some clue as to how the term was pronounced in earlier
stages of the language. These can, in turn, help explain the derivation of the
long-vowel readings.




The premise of your question – that in native Japanese words long "o" sounds
are always spelled by adding お rather than by adding う – is simply incorrect.
(Consider the verb [儲ける]{もうける} and the nouns [お父さん]{おとうさん} and [素人]{しろうと}, for
example.)

That said, the う that occurs in the volitional form ending does in fact derive
from the auxiliary verb む, as explained in the sources quoted in the post to
which you have linked.

