The closest relative of modern literary Latvian language - Latgalian language, common in Eastern Latvia (Latgale), which is also viewed as a dialect / variety, even though these languages are quite different story. In addition Latgalian language, has a close relationship Latvian and Lithuanian language. Until the mid XV, all the Baltic idioms, as well as East-Slavic languages were very close to each other, but the political factors and the formation of various government entities on the territory of the Baltic states led to the progressive fragmentation baltic range.
Initially, the Eastern Baltic languages split from the Western Baltic (or, perhaps, from a hypothetical Proto-Baltic language) between 400 and 600 mi-mi years. In doing so, carriers of Eastern Baltic languages (Prussians, Curonians) were subjected to severe germanization and the beginning of XVIII fully assimilated into the German language environment (as Polabska Slavs, as well as Masuria). Carriers of the Baltic languages have migrated to the territory of modern Latvia relatively late - between the VI-X vv n. e. Interestingly, by that time this territory, like the vast Russian plains have been inhabited by Finno-Ugric peoples, gradually blend into the baltic tribes. Remains of the Finno-Ugric area remained on the territory of modern Latvia in the form of so-called Livonian language, under threat of extinction since the beginning of XX century. To the north - on the territory of Estonia, the ancient Finno-Ugric origin subdialects preserved and evolved into the modern Estonian language.
The differentiation between Lithuanian and Latvian started in the 800-s, however, they still long to be dialects of one language. Intermediate dialects exist, at least until the XIV-XV centuries, and likely until the XVII century. It also had a significant influence on the development of independent language lesson Livonian Order in the XIII and XIV centuries River Western Dvina / Daugava (nearly coincides with the territory of modern Latvia). Otlichitelmi main factor in the Latvian language of the Lithuanian thus become the Finno-Ugric substrate in the early Middle Ages, as well as German and Eastern slave in the Middle Ages and the new time. Lithuanian language has experienced less German, but most of the West-Slavic influence.