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The Sindar of Beleriand were divided in several groups, and their language had developed some dialects. North Sindarin, the flavour of Sindarin spoken by the Mithrim, the northernmost group of the Sindar, differed from the Sindarin of Beleriand proper in many aspects. It was this language which was adopted by the exiled Noldor after their return to Middle-earth, and by their mortal allies, the House of Beor. During this time North Sindarin was changed much, partially due to the adoption of Quenya features, and partially due to the love of the Noldor for making linguistic changes. Beren's heritage was clear to Thingol of Doriath as he spoke the North Sindarin of his homeland.
North Sindarin retained many features of Archaic Sindarin which had been lost in the Sindarin of Beleriand proper, but also went through several changes of its own: lenition occurred far less in this dialect than in the other dialects.
After the end of the First Age, the survivors of Beleriand's realms generally adopted the more southern variants of Sindarin, but several proper names which are uninterpretable in normal Sindarin which remained in use during the Third Age show North Sindarin influence.
 Other versions of the legendarium
In Tolkien's earliest writings, there was a language called Ilkorin, the language of the Ilkorindi (the Dark Elves who stayed in Great Lands). After Tolkien evolved his mythology, the background and setting of the stories changed, and Ilkorin was replaced by Sindarin, the language of the Sindar.
Edward Kloczko observed that the few words mentioned as "Mithrim" fit to the phonological changes and principles seen in Ilkorin: for example Mithrim has ô where Sindarin has au, and retains final -v and -m where Sindarin has final -w; he theorized that Tolkien recycled parts of Ilkorin as Mithrim.
 The case of cw
This setting is part only of the later conception and Ilkorin, which belongs to earlier version, does not reflect this: primitive kw stays cw or c in some Ilkorin words such as alch (ALÁKwÂ), côm (KwAM), cwess (KwESS), salch (SALÁKwÊ) something that would be impossible in a Sindarin dialect.
Another problem with the identification of Ilkorin with North Sindarin is that the Ilkorin words which have survived in the canon as Sindarin, have little to do with the Mithrim.
In the early legendarium, the Ilkorindi were the vernacular Elves of all Beleriand, and therefore Ilkorin names are to be found in almost all the range of the stories. Names Tolkien marked as Ilkorin have to do with Doriath (Thingol, Beleg, Belthronding, Aros), Ossiriand (Lindon, Brilthor, Duilwen) or the Falas (Brithon, Brithombar).
Another example is the name Esgaroth, the lake in Rhovanion where Men dwelt in the Third Age, which Tolkien listed as Ilkorin; The Hobbit was written in 1937 while Tolkien still worked on The Etymologies.
The Mithrim on the other hand would hardly give names to those locations. There is little room of speculation of why a First Age Mithrimin dialect should reach Ossiriand, or even Rhovanion of the Third Age.
 Mithrimin and Doriathrin
The word lómin (cf. Dor-Lómin) was a variant of lómen "echoing", both in Doriathrin, another dialect of Tolkien's early legendarium; Doriathrin shows adjectives ending in -in instead of standard Sindarin -en (Dungorthin, muilin).
- ↑ J.R.R. Tolkien, "Words, Phrases and Passages in Various Tongues in The Lord of the Rings", in Parma Eldalamberon XVII (edited by Christopher Gilson) p.134
- ↑ J.R.R. Tolkien, Christopher Tolkien (ed.), The War of the Jewels, "Index"
- ↑ Edward Kloczko, "Ilkorin and North Sindarin", in Tyalië Tyelelliéva #9, October 1996
- ↑ J.R.R. Tolkien, Christopher Tolkien (ed.), The Lost Road and Other Writings, "Part Three: The Etymologies", entry LAM
- ↑ J.R.R. Tolkien, "Words, Phrases and Passages in Various Tongues in The Lord of the Rings", in Parma Eldalamberon XVII (edited by Christopher Gilson) p.133
- J.R.R. Tolkien, Christopher Tolkien (ed.), The War of the Jewels
- Ryszard Derdzinski, "Northern Dialect of Sindarin", Gwaith-i-Phethdain
- Helge Fauskanger, "Ilkorin", Ardalambion
- Roman Rausch, "Mithrimin", at Sindanórië
- Wordlist of Mithrim, comprised of Ilkorin corpus with neo-North Sindarin "updates"