According to Tolkien, Adûnaic “employs prefixions more frequently than suffixion” in word-formation (SD/417). This statement is at odds with the available evidence, however. The endings for noun cases are all suffixes, Adûnaic makes use of prepositional suffixes, and most inflections for verb tenses occur at the end of the word. The only clear prefixal formations are pronominal-prefixes and the genitive prefix an-.

It is possible that Tolkien was referring to the process of extending primitive roots. Primitive Eldarin extended roots by adding suffixal elements, such as the Elvish root √KAL “light; shine” which was extended to √KALAR “glory, radiance” (PE18/87). Perhaps Primitive Adûnaic extended roots by adding prefixes. If so, this word formation rule may refer instead to Primitive Adûnaic word formation, but we have no example of such extensions, either prefixal or suffixal.