North Sindarin is a dialect of Sindarin spoken in northern Beleriand where it was adopted by many of the Noldor, most notably the followers of the sons of Fëanor. Its biggest distinction is that it preserved intervocalic [m], which in ordinary Sindarin developed into [v] (PE17/128). This dialect did not survive the fall of Beleriand, but was preserved in a few ancient names such as Hithlum and Dor-lómin. As first suggested by Edward Kloczko in Tyalië Tyelelliéva #9 (October 1996), it appears that Tolkien preserved some aspects of his earlier Ilkorin language in this North Sindarin dialect. A reproduction of Kloczko’s article can be seen in the Ilkorin page on the Ardalambion website (bottom of the page).