ㅚ>ㅢ, ㅟ>ㅢ found around the 17th century have been explained as unrounding conditioning either [labial] or [round] so far, and it is clear that they involve the phonologic motive of ‘dissimilation’. However, ㅚ>ㅢ, ㅟ>ㅢ where we cannot find any phonologic conditions as in ‘반되>반듸>반디’(螢), ‘바퇴->버틔->버티-’ (柱), or ‘여위->여의-’(瘦, 渴) may not be referred to as unrounding just because they ended up becoming non-rounded vowels consequently. This study has revealed the characteristics of ㅚ>ㅢ, ㅟ>ㅢ of which phonologic motive does not seem clear.
ㅚ>ㅢ, ㅟ>ㅢ of which phonetic conditions are not clear are explained with assimilation resulted from j generated within the syllabic structure internally.
They are, at the same time, phonologic shifts generated sporadically in the process that the Korean vowel system embracing some blankness in the front complements such imbalance. In the process of monophthongization of /əj/, /aj/, /ə/, /a/ turn into variations [ej], [ɛj] through assimilation with /j/’s frontalization, but /o/, /u/ can be frontalized not only as [øj], [yj] but as /ɨ/, too, which is located right before according to the vowel system. In that period, ㆍ’s dephonologization generated variations in ‘:오’, ‘으:우’ roundedness contrasts, which allowed [oj]~[ɨj], [uj]~[ɨj] to be realized.