Serfdom became the dominant form of relation between Russian peasants and nobility in the 17th century. Serfdom most commonly existed in the central and southern areas of the Tsardom of Russia and, from 1721, of the subsequent Russian Empire. Serfdom in Ukraine, in other Cossack lands, in the Urals and in Siberia generally occurred rarely until, during the reign of Catherine the Great (r. 1762–1796), it spread to Ukraine and to the former Polish–Lithuanian Commonwealth; noblemen began to send their serfs into Cossack lands in an attempt to harvest their extensive untapped natural resources.