Monophthongization is the reduction of a diphthong to a long vowel. It is a gradual natural assimilatory phenomenon (assimilation) caused by the total conflation of the diphthong’s elements.

Evidence for the early monophthongization of the diphthongs /ei̯/ and /ou̯/ is found in various dialects: PotEdán = Poteidán ‘Poseidon’ (Corinth, 6th c. BCE; for Cor. <E> = /eː/, see del Barrio Vega 2010), katékhe = katékhei ‘(s)he detains’ (Ambracia, 6th c. BCE), tôto = toûto ‘tha…