The full forms of biconsonantal and triconsontal roots could be modified by placing the extra characteristic vowel as a prefix instead after the second consonant, for example: ✶izir < √ZIR or ✶aklab, ✶akalb- < √KALAB (SD/422). For prefixed triconsonantal words like ✶akalb-, the word must end in a basic vowel different from the characteristic vowel, such as: ✶akalbi, ✶akalbu (SD/422). This was because (a) Primitive Adûnaic words could not end in consonant clusters and (b) the characteristic vowel was not repeated more than twice.

Except for words like ✶akalb-, subordinate-vowel-variation could not be used with Primitive Adûnaic words that had a prefixed vowel. This is because (a) variant vowels could not appear as a prefix or in the normal position between the first and second consonant (as with ✶izir and ✶akalb-) and (b) variant vowels could not appear when the characteristic vowel is suppressed from its normal position (as with ✶aklab).

In theory, prefixed triconsonantal forms such as ✶akalib or ✶akalub were possible, but Tolkien gave no such examples. Most likely, this was because basic primitive word forms were limited at most two vowels (as implied but not explicitly stated on SD/422), with the sole exception of words like ✶akalbu.