The Krivichs or Kryvichs were a tribal union of Early East Slavs between the 6th and the 12th centuries. It is suggested that originally the Krivichi were native to the area around Pskov. They migrated to the mostly Finnic areas in the upper reaches of the Volga, Dnieper, Dvina, areas south of the lower reaches of river Velikaya and parts of the Neman basin.
A miniature from Radzivill Chronicle showing ancient tribe of Krivichs
Belarusian is an East Slavic language. It is one of the two official languages in Belarus, alongside Russian. Additionally, it is spoken in some parts of Russia, Lithuania, Latvia, Poland, and Ukraine by Belarusian minorities in those countries.
The Casimir's Code of 1468, in Ruthenian
The third Lithuanian statute of 1588, all three written in Ruthenian
Ruthenian Bible by Francysk Skaryna, 1517, first ever book printed in Eastern Europe
The cover of the copy of the Dictionary of the Belarusian Local Tongue by Ivan Nasovič preserved at the Francis Skaryna Belarusian Library and Museum