The semi-vowels [w] and [j] vanished medially when they were followed by their corresponding vowels [u] and [i], respectively (SD/419). Tolkien indicated they “disappeared prehistorically” in this position. They did, however, survive at the beginning of words (SD/419).

When the preceding vowel was the same as the following, this would result in a long vowel. Otherwise if the preceding vowel was an [a], the result would be one of the two primitive diphthongs [au] or [ai]. When the combinations were [ui] or [iu], however, the result was not a diphthong and the [w] or [j] was restored when the glide consonant [w] or [j] was inserted between [u] or [i] and a following distinct vowel.