Vowel fronting refers to a shift in the place of articulation of a vowel (i.e., the position of the highest point of the tongue during its pronunciation) to one further forward in the mouth. Technically any of the changes which involve the raising of a front vowel (e.g. that of  ē [ɛː] > [e̝ː] > [iː] in Koine) could also be classed as fronting, since heig…