The characteristic-vowel of a base always appeared at least once, but not every vowel in a primitive word needed to be the same as the characteristic vowel. If there was another vowel after the second consonant or suffixed to the end of the word, it could be one of the other two basic vowels (SD/423), for example: ✶Ad. √ZIR > ✶zira, ✶ziru or √KALAB > ✶kalib, ✶kalub, ✶kalbi, ✶kalbu. Tolkien called this process the “variation of subordinate vowels” (SD/424).

Such subordinate vowel variation could only occur when the characteristic vowel appeared in its normal position between the first and second consonants. Where this was not the case due to vowel-suppression, all of the word’s vowels had to be the characteristic vowel. Therefore, primitive forms such as **izru < √ZIR or **aklub < √KALAB were not permitted (SD/434). Furthermore, any prefixed vowel was always the characteristic vowel (SD/423).

In forms where the characteristic vowel already appeared twice, the third vowel was required to be a different vowel, so that ✶akalbu < √KALAB was possible but **akalba was not (SD/422). In all cases, the variant subordinate vowel (if any) was the final vowel.