S. medial [j] became [i]; [-j-] > [-i-]
In Sindarin and Noldorin any surviving medial -y- [j] became i at some stage. This is a factor in Sindarin orthography, where i always represents the vowel [i] medially, but at the beginning of a word before another vowel represents the “y”-sound [j], because it is the only place where this sound can appear:
Do you think péti "lips" could be a model how CV monosyllables form their plurals? I mean like pé - péti, ré - *réti, thé - *théti, possibly pí - *píti etc
It's a wild stab, I know, it just occurred to me.