Peter and Nicholas were two brothers who reigned as Tsars of Russia from 1825 to 1855. They are commonly referred to as the CZARS of Russia, a title that was used to refer to the monarchs of the Russian Empire. The term 'TSAR' is derived from the Latin word 'CAESAR', which was used to refer to Roman emperors. The term 'CZAR' is derived from the Slavic word 'TSAR', which was used to refer to the rulers of the Russian Empire.