Every Adûnaic root had a characteric vowel which, together with the root’s consonants, defined the base (SD/415). For examples, the roots ✶Ad. √ZIR and *√ZUR would be distinct roots, despite sharing all the same consonants. The second root *√ZUR in this example is not directly attested in Tolkien’s published works, but it could be the root of Ad. zôrî “nurse”, as suggested by Andreas Moehn (EotAL/ZUR).

The characteristic vowel of a root was always one of the three basic primitive vowels [a], [i] or [u] (SD/422). In derived words, the normal position of the characteristic vowel was between the first and second consonants (SD/422). It could also appear a second time in some other position, but rarely appeared more than twice (SD/422). With vowel-suppression, it could be removed from its normal position, though in such cases it always appeared elsewhere in the word.

There were no gradiations of vowels in Primitive Adûnaic that stood in etymological relationships, but the common vowel-modifications could produce effects that resembled gradiation (SD/415,425).