A word for “narrow neck, isthmus” in The Etymologies of the 1930s (Ety/YAK). Although Christopher Tolkien presented ᴹQ. yatta as if it were a derivative of ᴹ√YAK (LR/400), in their Addenda and Corrigenda to the Etymologies Carl Hostetter and Patrick Wynne indicated it was actually a derivative of a deleted root ᴹ√YATH, that appeared between ᴹ√YAK and ᴹ√YAT “join” (VT46/22). Although ᴹ√YATH was deleted, ᴹQ. yatta was not, and it was possible Tolkien reconceived of it as a derivative of ᴹ√YAT, as suggested by Hostetter and Wynne. As for ᴹ√YAK, Tolkien had a similar word ᴹQ. yat (yaht-) “neck” derived from that root.
Conceptual Development: In the Qenya Lexicon of the 1910s, Tolkien had ᴱQ. yatta “neck, also isthmus” under the early root ᴱ√ẎATA “join”, along with an archaic variant †yat (QL/105). In a list of body parts from the 1920s, Tolkien instead had ᴱQ. yat (yakt-) “neck” (PE14/117).
Neo-Quenya: It is not clear that both yaht- and yatta should coexist. For purpose of Neo-Quenya, I would just use yatta “(narrow) neck, isthmus”, as it has a clearer etymology and ᴹ√YAT has a larger set of derivatives.