These developments mirror the well-known changes of Noldorin, often called the a-affection or a-umlaut. Where an [i] or [u] appeared in a syllable before a primitive final [a] (including a-stem verbs), these vowels shift to [e] and [o] respectively. Since the final [a] usually disappeared (except in a-stem verbs), this sound shift can give a clue to the more primitive forms of words. Some clear examples of this change are ᴹ✶rista > Ilk. rest (Ety/RIS²) and ᴹ✶tultā- > Ilk. tolda- (Ety/TUL).

In Ilkorin, this sound change seems to be prevented or reverted in some cases: see the rule that [e], [o] became [i], [u] before [nn], [nd], [ŋg] for further discussion. Variations in the adjective suffix -en vs. -in (from ᴹ✶-inā) may also indicate some uncertainty on Tolkien’s part on the exact functioning of this rule.