Belarussian belongs to the eastern group of Slavic languages and is very similar to Russian. It did not become a separate language until the 15th century, when it was the official language of the grand duchy of Lithuania. It is written in the Cyrillic alphabet but has two letters not in Russian and a number of distinctive sounds. The vocabulary has borrowings from Polish, Lithuanian, German, Latin, and Turkic. Russian and other languages are also spoken.