Common Eldarin is the primordial tongue of the Eldar, those Elves who left for Valinor.
Common Eldarin split off Primitive Quendian, the original language of all Quendi, or Elves, when the Eldar left Cuiviénen, since their language still evolved during the Great March.
Since the Eldar dominate the annals of the Elder Days, all the Elvish languages that appear, come from that branch. The Avarin languages possibly followed a wholly different evolution.
Common Eldarin led is the last common ancestor of Quenya and the [Telerin languages.
Minor changes of the Primitive Quendian brought the state of Common Eldarin. These changes had to do with several developments of the word-forms, usually normalization. For example: