Most Primitive Adûnaic roots were triconsonantal (SD/415), such as ✶Ad. √KALAB “fall”. Tolkien said that “in this point Adûnaic shows affinity with Khazadian [Dwarvish] rather than Avallonian [Elvish]”. Any of the Primitive Adûnaic consonants could appear in the first, second or third position, but usually at least one of the last two consonants was a continuant (SD/417), so that it was rare for both of the final consonants to be stops. Since the contact of these last two consonants was a common way for consonant clusters to be formed, this meant that clusters of two stops was rare in Primitive Adûnaic (see medial-consonants).

The word forms of triconsonantal roots are tricky to puzzle out. On SD/418, Tolkien listed AK(A)LAB(A) and (A)KALBA, but elsewhere he said that “a basic form does not usually have the CV [characteristic vowel] repeated more than twice” (SD/422). Going by the examples he gave for vowel suffixion, prefixion, and suppression, we get the possible forms ✶kal(a)b, ✶kalb(a), ✶aklab and ✶akalb(u). In the first two patterns, the parenthetical vowel may differ from the characteric vowel by the rules of subordinate-vowel-variation, such as: ✶kalib, ✶kalub, ✶kalbi, ✶kalbu. In the fourth pattern ✶akalb(u), the last vowel was required to be different from the CV: ✶akalbi, ✶akalbu but not **akalba. See the discussion of the different vowel-variations for more information.

The form ✶akalb(u) was the only clear pattern with three vowels. Tolkien listed some additional three-vowel patterns on SD/425, of the form ✶kulubba and ✶kulumba, but these examples all involved consonant-doubling or nasal-infixion, and seem to be part of those formations. These patterns as well as ✶akalb(u) both had a suffixed non-characteristic vowel to avoid final consonant cluster, something that was not allowed with final-consonants in Primitive Adûnaic.

Though not explicity discussed by Tolkien, it appears that single-vowel triconsonantal forms were sometime possible, such as the example ✶dāw’r. See the discussion under the single-vowel-form for more information.

This gives us five basic patterns: ✶kal(a)b, ✶kalb(a), ✶aklab, ✶akalb(u) and ✶daw’r, the last of which was probably only possible when the middle consonant was one of the semi-vowels [w] or [j]. From these basic patterns, the root could undergo further root-modifications.

Tolkien was somewhat inconsistent in the ways he represented unmodified triconsonantal roots. In some cases, he wrote these forms as LAW’M (SD/415) and GIM’L (SD/434), with the characteric vowel only in its normal position between the first and second consonant. In most cases, though, he used forms such as KALAB or GIMIL with the characteristic vowel after both the first and second consonant. The discussions here adopts the two-vowel form for its representation of these roots.