Tolkien used a number of similar roots as the basis for “line” words throughout his life. The earliest of these appeared in the Qenya Lexicon as ᴱ√TEHE [teχe] “pull” (gloss marked with a “?” by Tolkien) with derivatives like ᴱQ. tea “straight”, ᴱQ. telya “attractive; importunate”, and ᴱQ. tie “line, direction, route, road” (QL/90), the last of these surviving more or less unchanged all the way into the published version of The Lord of the Rings (LotR/377). The early root ᴱ√TEHE also had derivatives in the contemporaneous Gnomish Lexicon such as G. tê “mark, line; track; path”, G. tî or tion “straight”, and G. tîr “honest; esteem, regard, honour”, originally “straight, upright” (GL/69, 71). Primitive ᴱ✶tegna > ᴱQ. tína/ᴱN. tain “straight” from Early Noldorin Word-lists from the 1920s may represent a shift in the form of the root to ✱ᴱ√TEGE (PE13/153, 165).
In The Etymologies of the 1930s the root appeared as {ᴹ√TEƷ >>} ᴹ√TEÑ “line, direction” with derivatives like ᴹQ. tie/N. tê “line, way” and ᴹQ. téra/N. tîr “straight, right” (Ety/TEƷ, TEÑ). In the Outline of Phonology Tolkien gave √TEG “line”, whereas √TEÑ was given as the basis for Q. tenna “a thought, notion, idea” and thus clearly with a different meaning; see the entry √TEÑ for further discussion. In any case it is clear that Tolkien considered various ancient velar consonants for the second consonant of this root, all ultimately vanishing in the child languages with similar vocalic effects: 1910s teχ-, 1920s teg-, 1930s {teʒ- >>} teñ- and 1950s teg-.
Neo-Quenya: For purposes of Neo-Eldarin, I would assume this root was √TEƷ in order to preserve the validity of the word téma “series”. This is because Tolkien kept vacillating on the development of gm, but usually had gm > ngw, so that teg-mā > tengwa, not téma.
This root first appeared as unglossed ᴹ√PATH in The Etymologies of the 1930s with derivatives like ᴹQ. pasta/N. path “smooth” and N. pathw “level place, sward” (Ety/PATH). It reappeared in the Outline of Phonology from the early 1950s as √PATH “to smooth” (PE19/88), and again in etymological notes from 1959 as √PĂTH/PAS “smooth (to feel), silky” (PE17/171).