Eldarin is the Elvish language family that includes Quenya and Sindarin in J.R.R. Tolkien’s works.
Tolkien constructed several Elvish languages with related histories rather than treating Elvish as one uniform tongue. Quenya and Sindarin are the two most extensively developed varieties. Both ultimately belong to the linguistic traditions of the Eldar, the Elves who undertook the Great Journey toward Valinor.
Quenya is associated especially with the High Elves and preserves an elevated, classical status in much of the later history. Sindarin became the everyday Elvish language of many Elves in Middle-earth. Their relationship is comparable to related languages that have diverged over time, not simply to two accents of the same speech.
Tolkien’s language construction drew on his professional interest in historical linguistics. Sound changes, vocabulary, grammar, scripts, and fictional migration histories all contribute to the impression that Middle-earth’s languages developed through real historical processes.