The vowels a, ê and ô readily combine with a following vowel into diphthongs, but the vowels u and i could not appear initially in a diphthong (SD/423-4). Instead, a glide-consonant is inserted between the u or i and a following distinct vowel: w after u and y after i. As Tolkien states: “in composition or inflexion a ‘glide’ W was developed between U and a following vowel (other than U) and this developed into a full consonant in Adunaic; similarly Y developed between I and a following vowel (other than I)” (SD/434).

The technical linguistic term for such addition of consonants is “excrescence”, while the term “glide consonant” is simply another term for semi-vowel. The primary form of excrescence in Adûnaic is such an addition of a glide consonant, so the discussion here uses this less technical term to describe the process.

Examples of both glide-consonants appear in the Lament of Akallabêth (SD/247). In the phrase Ar-Pharazônun azaggara Avalôiyada, the final word Avalôiyada “against the Powers” is a combination of the plural Avalôi of Avalô “Power, God” and the suffix -ada “toward, against”, with a glide consonant “y” between them. In the phrase Bârim an-Adûn yurahtam dâira sâibêth-mâ Êruvô, the final word Êruvô “from Eru” is a combination of Êru and the suffix -ô “of, from” with the glide consonant “v” (pronounced [w]), where the w-sound is represented orthographically by “v” as sometimes happened (SD/434).

Glide-consonants also appear in various example noun declensions. For example, when the dual suffix -at was added to the nouns gimli “star” and nîlu “moon”, the result was gimliyat and nîluwat (SD/431).

Archaic Glide Consonants with -ê and -ô

It seems that archaically a final -ê or -ô could produced something like a glide consonant, decomposing into -ay- or -aw- when followed by an a or u (in the case of ê) or an a or i (in the case of ô). More accurately, the primitive diphthongs ✶ai and ✶au were preserved in these case, the second vowel becoming the semi-vowels y and w between the a and the following (distinct) vowel:

✶ai-a	>	†aya		✶au-a	>	†awa
✶ai-u	>	†ayu		✶au-i	>	†awi

If the last vowel was not distinct, the development would be:

✶ai-i	>	†êi	>	ê
✶au-u	>	†ôu	>	ô

As evidence for these formations, there are archaic inflections for izrê “sweetheart, beloved” (SD/438): †izrâyu (objective singular) and †izrayat (normal dual) [it is not clear why the a is long in izrâyu; perhaps it is a slip or a misreading]. Similarly, there is an archaic dual form †manawat of manô “spirit” (but no variant objective, since ✶au-u > ô). Tolkien indicates these are all archaic forms, however, so it is more likely that in Classical Adûnaic the vowels were simply absorbed as described under vowel-combinations.