✶Ad. root-modifications grammar.
Once the basic pattern of a primitive word was established by vowel-variations, it could undergo further modifications to produce the final form of the word. There were two kinds of further modifications: vowel-modifications (vowel-lengthening and a-fortification) and consonant-modifications (nasal-infixion and consonant-doubling) (SD/424).
Tolkien states that “any given derivative never shows two of the one [i.e. same] kind of variation at the same time” (SD/424), where the vowel modifications were one kind of process and the consonant modifications another. Thus, any given word would have at most two modifications: one vowel modification and one consonant modification. Tolkien does list a few examples where both kinds of modifications occur, such as ✶kullūb < √KULUB (SD/425) with both consonant doubling and vowel lengthening.
Starting from the basic biconsonantal patterns (✶zir(i), ✶izri, ✶izir) and triconsonantal patterns (✶kal(a)b, ✶kalb(a), ✶aklab, ✶akalb(u) and ✶daw’r), there were a large number of possible words could be produced through these further modifications, especially when subordinate-vowel-variation was included. For any given root, only a few of the possible variations were realized as actual words, however (SD/425).
References ✧ SD/424-425