Where a-fortification and vowel contacts produced the primitive diphthongs ✶[au], ✶[ai], these monophthongized into the simple long vowels [ē] and [ō] (SD/423). This was the means whereby these vowels were introduced into Adûnaic. This rule only applied to short diphthongs, since later long diphthongs survived unchanged (SD/423). According to Tolkien, the primitive diphthong were only found “in the oldest texts”, so this must have been an early change, though the rule seems to have remained functional throughout the development of Adûnaic.