A root appearing in various notes written around 1957 connected to the mind (PE17/145, 155, 189; UT/400; VT43/16). Its most notable derivatives are Q. indo/S. ind “(state of) mind”. In The Etymologies of the 1930s, ᴹQ. indo “heart, mood” and N. ind “inner thought, meaning, heart” were instead derived from the root ᴹ√ID; this 1930s root was unglossed but had derivatives connected to both thoughts and desires, such as ᴹQ. íre “desire, longing” (Ety/ID).
Tolkien introduced a new derivation of indo from √IN(ID) in Quenya Notes written in 1957 (PE17/145, 189):
> IN-I-D, mind, inner thought. These refer to the movements or activities of the fëa or “spirit” (rational soul). indo, inner thought, in fea as exhibited in character or [?personality]. indóme, settled character, also used of the “will” of Eru (PE17/189).
The root √IN- “inmost heart, thought, mind” with extended forms √INID and √INIS also appeared in contemporaneous Notes on Names (NN), again as the basis for indo (PE17/155).
Tolkien coined yet another etymology of Q. indo in notes written in 1969, deriving it from √NID “force, press(ure), thrust” (PE22/165). This new derivation doesn’t necessarily invalidate √INID, but rather gives a new basis for the root itself as an vocalic augment of √NID rather than an extended form of √IN-.
This and similar roots were used for Elvish words for “one” for most of Tolkien’s life, though generally in competition with √ER. The root first appeared as unglossed ᴱ√MĪ in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. mir “one” and ᴱQ. minqe “eleven”; the parenthetical comment (mi’i) indicated it originally had some other now-lost consonant, though Tolkien marked this comment with a “?” (QL/61). It also had derivatives in the contemporaneous Gnomish Lexicon such as G. min “one single” (GL/57).
The Qenya Lexicon of the 1910s had a distinct root ᴱ√MINI from which “tower” words were derived, such as ᴱQ. mindon/G. mindon (QL/61; GL/57). In The Etymologies of the 1930s these roots were merged together into a single root ᴹ√MINI “stand alone, stick out”, whose derivatives included both ᴹQ. min/N. min “one” as well as ᴹQ. mindo and N. mindon/minnas “tower” (Ety/MIN).
In Tolkien’s later writings √MIN was mostly mentioned in the context of Elvish numbering systems, and given glosses like “one” or “first of a series” (VT42/24; VT47/16-17), but in notes written in 1967-69 Tolkien indicated it was still the basis for “tower” words, most likely from the sense “prominent” (VT42/24). In these later notes Tolkien was careful to distinguish √MIN “one” used in counting vs. √ER “one” used for individual things: √MIN was for the first of a series of things, whereas √ER was for a single thing that was unique, alone or in isolation. The distinct senses of these two roots dates back at least as far as The Etymologies of the 1930s (Ety/ER, MIN).