This root was the basis for Elvish “love” words for all of Tolkien’s life. The root first appeared as ᴱ√MELE “love” in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. mel- “to love”, ᴱQ. meles(se) “love”, and ᴱQ. melin “dear, beloved” (QL/60). In the contemporaneous Gnomish Lexicon it had similar derivatives like G. mel- “love” and G. melon “dear, beloved” (GL/57).
In The Etymologies of the 1930s Tolkien specified that ᴹ√MEL meant “love (as friend)”, and for the first time it included the derivative N. mellon “friend” (Ety/MEL); Gnomish “friend” words from the 1910s were mostly based on G. ged (GL/38). However, the same entry included ᴹQ. melindo/ᴹQ. melisse “lover” (male and female), so it seems even in the 1930s it could refer to romantic love (Ety/MEL). The root continued to appear in Tolkien’s later writings associated with “love” (PE18/46, 96; PE17/41; VT39/10).
In notes from 1959, Tolkien elaborated on the precise sense of √MEL and its role in romantic and non-romantic love:
> Love, which Men might call “friendship” (but for the greater strength and warmth and permanency with which it was felt by the Quendi) was represented by √mel. This was primarily a motion or inclination of the fëa [“spirit”], and therefore could occur between persons of the same sex or different sexes. It included no sexual or procreative desire, though naturally in Incarnates the difference of sex altered the emotion, since “sex” is held by the Eldar to belong also to the fëa and not solely to the hröa [“body”], and is therefore not wholly included in procreation ... The “desire” for marriage and bodily union was represented by √yer; but this never in the uncorrupted occurred without “love” √mel, nor without the desire for children. This element was therefore seldom used except to describe occasions of its dominance in the process of courting and marriage. The feelings of lovers desiring marriage, and of husband and wife, were usually described by √mel. This “love” remained, of course, permanent after the satisfaction of √yer in the “Time of the Children”; but was strengthened by this satisfaction and the memory of it to a normally unbreakable bond (NM/20).
Thus √MEL was close in sense to Greek “philia”, used of friendship, whereas √YER was used of “eros” or sexual desire. But in Elvish thinking, √MEL was essential for romantic love, and √YER only arose from that. Furthermore, √YER was not the most important element in the love between romantic partners, as the period of procreation and child-rearing took up a relatively small portion of Elvish lives. It was the more enduring feeling of friendship between lovers that really mattered, and thus √MEL was used of both non-romantic and romantic love, though it had not particular sexual connotation.
This root was the basis for Elvish “son” words for much of Tolkien’s life. The earliest indications of this root are ᴱQ. †Y̯ó (or y̯ond-) “son” and ᴱQ. yondo “male descendant”, both tied to the patronymic suffix ᴱQ. -ion “son of, descendant of” appearing in many names (QL/106). In the contemporaneous Gnomish Lexicon there was also the related patronymic prefix G. go- (GL/40), which implies the existence of a root ✱ᴱ√YO(NO) since [[g|initial [j] usually became [g]]] in Gnomish. However, go- was deleted and changed to G. bo-, along with new Qenya forms ᴱQ. vô and ᴱQ. vondo (GL/23, 40), implying a change to a root ✱ᴱ√VO(NO).
In Early Noldorin Word-lists from the 1920s, ᴱN. gó “son” reappeared along with ᴱQ. ion and yondi (PE13/144). In The Etymologies of the 1930s the root appeared as ᴹ√YŌ or ᴹ√YON “son” with derivatives like ᴹQ. yondo/N. ionn “son” and patronymic -ion (Ety/YŌ). The root reappeared in notes from the late 1940s with the sense “son or young man”, though more anciently a general masculine suffix (PE23/87).
However, in notes labeled “Changes affecting Silmarillion nomenclature” from the late 1950s, Tolkien wrote “Delete entirely yondo = ‘son’! Very unsuitable” (PE17/43). This particular note was rejected when Tolkien changed √YON “wide, extensive” to √YAN (PE17/42). Other notes in the same bundle indicate Tolkien was still seeking a new word for son, saying “Q wanted: son, daughter”, though yon(do) remained among the forms he was considering (PE17/170, 190). It seems Tolkien eventually stopped vacillating and restored √YON, since the patronymic -ion was never discarded, and yon- was the basis for “son” words in notes from the late 1960s (VT47/26).