Matt Yglesias dares to wander into Tolkien territory and quickly shows his Nube status. By that standard, the most relevant J.R.R. Tolkien passage comes from Appendix A of the Return of the King: There were three unions of the Eldar and the Edain: Lúthien and Beren; Idril and Tuor; Arwen and Aragorn. By the last the long-sundered branches of the Half-elven were reunited and their line was restored. It helps to recall here that Eldar is another word for elf and Edain is another word for human. Tolkien is saying here that there were two human-elf pairings in the backstory to the Lord of the Rings. One between Lúthien and Beren and another between Idril and Tuor. Both Arwen and Aragorn are descendants of one of these pairings. So when they get together in the course of the series, they reunite the half-elven lines. What a nube. Eldar is not just another word for Elf. The Eldar are the high elven who have lived in the land of the Valar. And Edain is not just another word for human.