The Baltic languages belong to the Balto-Slavic branch of the Indo-European language family, and are spoken by the Balts. Baltic languages are spoken mainly in areas extending east and southeast of the Baltic Sea in Northern Europe. Scholars usually regard them as a single language family divided into two groups: Western Baltic (containing only extinct languages), and Eastern Baltic (containing two living languages, Lithuanian and Latvian). The range of the Eastern Balts once reached to the Ural mountains. Although related, the Lithuanian, the Latvian, and particularly the Old Prussian vocabularies differ substantially from one another and are not mutually intelligible. Old Prussian (a Western Baltic language which went extinct in the 18th century) ranks as the most archaic of the Baltic languages.