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There are various sound changes that have occurred in Japanese that are known as "euphonic changes" (音便). These changes are actually multi-step processes which can be summarized by one change necessitating a solution to ultimately make the word "easier" to pronounce. In this installation, we will be learning about ウ音便, in which the final step is the mora affected being pronounced as /u/*. 

Pronunciation Note: In Standard Japanese, the vowel /u/ is pronounced as [ɯ], but this is a sound change that affected Eastern Japanese dialects and prevented ウ音便 from being as prevalent. So, for historical accuracy, [ɯ] will only be used to refer to Eastern Japanese pronunciation and [u] will be used elsewhere.

Notation Note: Symbols from the International Phonetic Alphabet are used in this lesson. If any symbol is unfamiliar to you, review Lessons 393-4. 

ウ音便 Due to Close Vowel Deletion

Step 1: ウ音便 first began by the close vowel (狭母音) /u/ or /i/ getting dropped. In the process, the roundness ( ◌ʷ ) of /u/ or palatalizing effect (◌ʲ) of /i/ is transferred over to the consonant in the same mora. 

Summary: Drop /u/~/i/, transfer roundness/palatalization to preceding consonant. 

Step 2: In the case of velar (軟口蓋音) morae /ku/ and /gu/, they manifest as /kʷ/ and /gʷ/ respectively. When the consonant weakens, becoming an approximant (接近音), the velar articulation is lost. In doing so, the consonant of the following morae becomes voiced in the case of /gu/. 

Summary: /kʷ/ and /gʷ/ → /w/~/j/ (true nature of approximant lost to time). Voicing of /g/ transferred to following mora. 

Step 2: In the case of (nasal) bilabial morae - /bi/, /bu/, /mi/, /mu/ - with the consonants /b/ and /m/ rendered as an approximant, their roundness articulation is still maintained, but the quality of /u/~/i/ also likely affected how this approximant was pronounced. What's more, because /m/ is nasal, its nasality would have been mapped onto this approximant. 
Summary: /bʷ/ → /w/~/j/, /mʷ/ → / j̃ /~/w̃/

※In the event that a nasal consonant follows in the next mora, some words deviate at Step 2 and exhibit a change to /N/ (a parallel phenomenon known as 撥音便).
※While the close vowel is dropped in Steps 1-2 and the consonant weakens, to maintain the moraic structure of Japanese and prevent consonant clusters, a schwa-like vowel may have been naturally inserted. 

Step 3: The approximant changes to /u/~/i/※. 

Step 4: This /u/ is then subject to fusing with the vowel in the preceding mora if said mora exists. This diphthong reduction is known as 直音化, and when /u/ ends up after /a/, it results in [o:]. 

※Instances of the approximant becoming /i/ are known as イ音便.

ウ音便 Due to Deleting Medial /w/: (/ɸu/ →) /wu/ → /u/

Another situation that is referred to as ウ音便 is what happened when medial /ɸ/ changed to /w/. If the vowel that follows was either /u/ already or became /u/ as a consequence or by happenstance, then /w/ would drop out due to /wu/ being an invalid mora. The result would be a mora that has been simplified to /u/. Likewise, if the consonant was already /w/ and its vowel changed to /u/, this would trigger the simplification of the mora to /u/. 

Unorthodox ウ音便: Inserting /u/

Another phenomenon often referred to as ウ音便 that does not result from deleting a sound like in the situations above involves inserting /u/ into a word to seemingly make it easier to pronounce. Examples include 八日 (/yaka/ → yauka/ → /yo:ka/) and 夫婦 (/ɸuɸu/ → /ɸu:ɸu/.

• 麹: /ko:ʑi/ derives from the nominal form of the verb kamosu 醸す (to brew sake). The medial /mo/ in /kamosi/ was dropped, but the voicing carried over to the next mora, creating: /kauⁿzi/. Later, the diphthong /au/ simplified to /o:/. 
• 手水: This is known for meaning "water for washing one's hands," especially at a shrine. The reason for why it is pronounced as /tɕo:zɯ/ today is due to ウ音便. Originally, it was /temiⁿzu/, but the medial /mi/ reduced to /u/, creating: /teu ⁿzu/. Then, by the diphthong /eu/ simplifying to /o:/, the initial consonant palatalized to produce its modern pronunciation. 
• 日向: This word is famous for being the only native word to contain /çɯ > hʲu/, and this too is thanks to ウ音便. Its original pronunciation would have been /pimuka/. Over time, the /p/ became /ɸ/ and then later turned into /h/. As for the ウ音便, the medial /mu/ simplified to /u/, and the juxtaposition of /i/ and /u/ naturally caused palatalization, giving its modern pronunciation of /hʲuga/. 
• 笄: /ko:gai/ meaning "hairpin" might not be recognizable as a compound word, but it is actually derived from 髪掻き, which would clearly be read as /kamikaki/. When the medial /mi/ reduced to /u/, the voicing moved over to the next mora, then the diphthong /au/ simplified to /o:/, producing the modern pronunciation.   
• 箒: The modern pronunciation of /ho:ki/ (broom) derives from /papaki/. Even in some dialects to this day, this can be recognized as the nominal form of an alternative form of the verb to sweep - 掃わく. Going ahead with changing /p/ to /h/ for simplicity, once the second /ha/ changed to /u/, the resulting diphthong /au/ simplified to /o:/, producing the modern pronunciation.
• 芳しい・馨しい has three valid pronunciations, each featuring a different sound change: /ko:baɕi:/ (featuring ウ音便), /kam.bashi:/ (featuring 撥音便), and /kagɯwaɕii/ (featuring the medial /ɸ/ to [w] sound change which occurred in Middle Japanese). The word in any case is a combination of 香 /ka/ meaning "fragrance" and an archaic adjective 美（くは）し meaning "delicate/beautiful." When the medial /gu/ was reduced to /u/, the voicing carried over to the next mora and the resulting diphthong /au/ was then simplified to /o:/. In the second reading, the medial /gu/ became simplified to /N/ but still caused voicing onto the next mora due to assimilation. 
• 申す: /mo:su/ was once pronounced as /mawosu/. When /wo/ reduced to /u/, the resulting diphthong /au/ simplified to /o:/, producing the modern pronunciation.
  
  ※Another sound change omitted in these descriptions is an intermediary pronunciation of /au/. Before becoming /o:/, it was actually pronounced as / ɔː/.This sound was referred to as 開音. Likewise, the simplification of other diphthongs to /o:/ were called 合音.

■ウ音便 in ワ行五段 Verb た形・て形 Conjugations

When learning how to conjugate the た形 and て形, you learned about 促音化 in which the final mora of the 連用形 of many 五段 verbs is simplified to a geminate consonant with the following consonant /t/. 

Ex. 会う /aɯ/ → 会って /atte/  

If we respell this verb using old Kana orthography and retrace this sound change, we get:

What's intriguing is that this use of 促音化 was a key feature of Eastern Japanese dialects, but this did not occur for ワ行五段 (W-Column Godan Verbs) in Western Japanese dialects. 

Instead, the medial h-mora reduced to /u/ via ウ音便. If we were to use the same verb 会う, this would produce 会うて (o:te). 

※These verbs typically exhibit 促音化 in current speech, but their ウ音便 forms are still prevalent to a degree. Verbs not marked are not said to exhibit 促音化, or at least not yet. 
† indicates words that have fallen out of use but are still considered valid words in Modern Japanese. 　

8.  戴天山の道士を訪（と）うて遇わず。
I came to visit the Taoist master at Daitianshan yet didn't meet him. 

9. 家にそうた鉄柵ごしに立派な庭園が見える。
A marvelous garden can be seen through the iron fence along the house.

10. 運輸大臣は、責任を負うているのか。
Does the Minister of Transport bear responsibility?

11. 政府ははっきり国民に問うた方がいいと思う。
I believe that it's best that the government clearly ask the people.

■ウ音便 in Volitional Forms

The modern volitional forms are also a product of ウ音便. The auxiliary verb for volition in Classical Japanese was ～む, but once this became ～う, it resorted in forms that we use today. 

Another situation of ウ音便 in Standard Japanese can be found when attaching ～ございます to adjectives. The /ku/ in the 連用形 simplifies to /u/ and follows the same rules as we've learned. 

However, in Western dialects, this affected adjectives in general in the same way it did for verbs. This means that the て形 of adjectives undergo this process as it follows the 連用形. Also just like verbs, any resulting /o:/ are subject to being shortened to /o/. 

12. 写真も何ものうてすんまへん。
Sorry for not having photos or anything.

13. 生きるっちゅうのはしんどうておもろい。
Living is tiring but also fun.