A root Tolkien introduced in the late 1960s as the basis for his latest Elvish word for “ten”: Q. quëan/quain, S. pae, T. pai(n) (VT42/24; VT48/6). It was an extension of √KWA “complete” as in “a complete set of (10) fingers”. Prior this late change, the usual word for “ten” was ᴹQ. kainen (along with other variants beginning with kai- or kea-) from the root ᴹ√KAYAN or ᴹ√KAYAR as it appeared in The Etymologies of the 1930s (Ety/KAYAN). This basis for “ten” dates back to the Early Qenya Grammar of the 1920s (PE14/49, PE14/82). Tolkien was still considering √KAYAN for “10” in the late 1960s before replacing it with √KWAY(AM) (VT48/12).
Primitive elvish
kay
root. lie, lie, [ᴹ√] lie down; [ᴱ√] rest, dwell
kayan
root. ten
kway
root. ten
kait-a
verb. lie, be on the ground
Tolkien used this root for “lie (down)” for most of his life. It appeared as ᴱ√KAYA “lie, rest; dwell” in the Qenya Lexicon of the 1910s (QL/46), but in that document there was another root ᴱ√KAMA “lie down”, with the derivative ᴱQ. kama- “to lie down” (QL/44). There is no sign of ᴱ√KAMA¹ being used this way after the 1910s, and in the 1920s Early Qenya Grammar, the verb for “lie down” was kaita- (PE14/58), which in the Qenya Lexicon of the 1910s had the transitive sense “to place” (QL/44).
The root ᴹ√KAY “lie down” reappeared in The Etymologies of the 1930s (Ety/KAY), and it appeared again in the Quenya Verbal System of the 1940s as the basis for the verb ᴹQ. kaita- “to lie (down)” (PE22/126). The root and associated Quenya verb continued to appear in Tolkien’s later writings all the way up through the late 1960s, always with the sense “lie” (PE17/72; PE22/156).