[citation needed] A type of i-mutation is also observed in Anatolian languages, including Hittite, Luwian, Lycian and Lydian.
[4][5][6][7] In Middle Korean, I-backward-sequenced vowels (ㅐ, ㅔ, ㅒ, ㅖ, ㅚ, ㅟ, ㅢ) were diphthongs, i.e. ㅐ [ɐj], ㅔ [əj], ㅒ [jɐj], ㅖ [jəj], ㅚ [oj], ㅟ [uj], ㅢ[ɯj ~ ɰi].
However, in early modern Korean, they are monophthongized by umlaut, i.e. ㅐ [ɛ], ㅔ [e], ㅒ [jɛ], ㅖ [je], ㅚ [ø], ㅟ [y] with only one exception: ㅢ.
Assimilation occurs when ㅣ is in front of (forward) or behind (backward) the syllable.
In standard Korean, only a few words are allowed to assimilate, however, exceptions are often observed in some dialects and casual usage.