The Tsardom of Rus' (Russian: Царство Русское) was the official name for the Russian state between Ivan IV's assumption of the title of Tsar in 1547 and Peter the Great's foundation of the Russian Empire in 1721. The name originated from the fact that it contained all of the Rus lands that were at the time free of foreign states domination. Some Western sources refer to this state as Muscovite Russia or Muscovy, the term originally applied to its predecessor, the Grand Duchy of Moscow.