When deriving a primitive word from a root, one of its isolated consonants could be modified by doubling it. This was usually the medial or final consonant, but sometimes the initial consonant could be doubled (SD/417), though (most likely) only when there was a prefixed vowel. At most one consonant in a word could be doubled (SD/417).

Where this consonant-doubling was applied finally, the result was a consonant cluster. Since clusters could not appear as final-consonants in Primitive Adûnaic, an additional vowel was suffixed to the word: ✶Ad. √KALAB > ✶kalabbi, ✶kalabbu (SD/425). Where the other two vowels were characteristic-vowels, this added vowel must necessarily be non-characteristic. Otherwise, the suffixed vowel could be characteristic or non-characteristic: ✶kalib → ✶kalibba, ✶kalibbi, ✶kalibbu (SD/425).

According to Tolkien, consonant doubling was originally in competition with nasal-infixion for various grammatical functions in Primitive Adûnaic (SD/417). However, later phonetic developments meant that many infixed nasals became voiceless stops, making it unlikely that nasal-infixion was used grammatically by the time of Classical Adûnaic. Consonant doubling was used in the formation of the Classical Adûnaic past tense, though.