Russian SFSR vs Russian Soviet Federative Socialist Republic

Russian SFSR

name
  • Russian Soviet Federative Socialist Republic, the name for modern day Russia before the collapse of the Soviet Union (1917-1991). 

Russian Soviet Federative Socialist Republic

name
  • The official name for modern-day Russia under the Communist Party from 1936 to 1991 (from 1918 to 1937, according to the Constitution of the RSFSR, called the Russian Socialist Federative Soviet Republic). Part of the USSR from 1922 to 1991. 

How often have the words Russian SFSR and Russian Soviet Federative Socialist Republic occurred in a corpus of books? (source: Google Ngram Viewer )