The term serf, in the sense of an unfree peasant of tsarist Russia, is the usual English-language translation of krepostnoy krest'yanin (крепостной крестьянин) which meant an unfree person who, unlike a slave, historically could be sold only together with the land to which they were "attached". However, this stopped being a requirement by the 19th century, and serfs were practically indistinguishable from slaves. Contemporary legal documents, such as Russkaya Pravda (12th century onwards), distinguished several degrees of feudal dependency of peasants. While another form of slavery in Russia, kholopstvo, was ended by Peter I in 1723, the serfdom (krepostnoe pravo) was abolished only by Alexander II's emancipation reform of 1861. 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 Little Russia (parts of today's central Ukraine), and 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; noblemen began to send their serfs into Cossack lands in an attempt to harvest their extensive untapped natural resources. Only the Russian state and Russian noblemen had the legal right to own serfs, but in practice commercial firms sold Russian serfs as slaves – not only within Russia but even abroad (especially into Persia and the Ottoman Empire) as "students or servants". Those "students and servants" were in fact owned by rich people, sometimes even by rich serfs, who were not noblemen. Emperor Nicholas I banned the trade of African slaves in 1842, though there were almost no Russians who participated in it, but Russian serfs were still sold and bought. Emperor Alexander I ( 1801 - 1825) wanted to reform the system but moved cautiously, liberating serfs in Estonia (1816), Livonia (1816), and Courland (1817) only.