In theory, the definition includes both and but Ancient Greek, which the term was originally created to describe, displays only the former, since the process is part of long-vowel shortening.
In the Attic and Ionic dialects of Ancient Greek, ēo and ēa often exchange length, becoming eō and eā.
The Homeric form of the genitive singular in the masculine first declension sometimes undergoes quantitative metathesis:[2] The Attic genitive singular Πηλεΐδ-ου Pēleḯd-ou uses a copy of the second-declension ending, which came from the same original form as the ending -oio (used in Homer)[4] — o-syo, thematic vowel o and case-ending -syo).
Sometimes this is quantitative metathesis:[5] But sometimes, when a long vowel occurs in the ending, ē is shortened to e without an accompanying lengthening of the vowel in the ending (but ou changes to ō to follow the other forms):[7] Some third-declension nouns had, in Proto-Indo-European, stems in -u or -i in zero-grade, -ew or -ey in short e-grade, and -ēw or -ēy in long ē-grade.
Some forms exemplify the quantitative-metathesis type of shortening: The accent of the genitive singular of the last two words violates the rules of accentuation.