A river in Beleriand flowing in the Sirion. It was spelled Teiglin in the published version of The Silmarillion (S/120), but Christopher Tolkien later said that the form should have been Taeglin(d) (WJ/309-310). This name was a combination of taeg “boundary” and lind¹ “singer” (WJ/309), so perhaps meant “*Boundary Singer”.
Conceptual Development: In Silmarillion drafts from the 1930s, this name first appeared as Ilk. Taiglin (SM/127, LR/260), and in The Etymologies was designated and Ilkorin name with the translation “Deep-pool” (EtyAC/TĀ, Ety/LIN¹). It remained Taiglin in the initial Silmarillion revisions from the 1950s-60s (WJ/49), but was later revised to Teiglin (WR/223, 228 note §28). Later still Tolkien devised a new form and etymology for this name: Taeglin(d), as noted above.
Christopher Tolkien was unaware of this final form when he was preparing The Silmarillion for publication, which is why he used the form Teiglin. In the Silmarillion Appendix, he made a reference to its earlier, Ilkorin derivation (SA/lin¹). It wasn’t until he was working on The History of Middle Earth series that he discovered his father’s notes on the form Taeglin.