The history of the Slavic languages stretches over 3,000 years, from the point at which the ancestral Proto-Balto-Slavic language broke up (c. 1500 BC) into the modern-day Slavic languages. The first 2,000 years or so consist of the pre-Slavic era; a long, stable period of gradual development during which the language remained unified, with no discernible dialectal differences. The last stage in which the language remained without internal differences can be dated around 500 AD and is sometimes termed Proto-Slavic proper or Early Proto-Slavic. Following this is the Common Slavic period (cc. 500–1000 AD), during which the first dialectal differences appeared but the entire Slavic-speaking area continued to function as a single language, with sound changes tending to spread throughout the entire area. By around 1000 AD, the area had broken up into separate East Slavic, West Slavic and South Slavic languages, and in the following centuries it broke up further into the various modern Slavic languages of which the following are extant: Belarusian, Russian, Rusyn and Ukrainian in the East; Czech, Slovak, Polish, Kashubian and the Sorbian languages in the West, and Bulgarian, Macedonian, Bosnian, Croatian, Montenegrin, Serbian and Slovenian in the South.