There were three editions. The first edition of 65 volumes (65,000 entries, plus a supplementary volume about the Soviet Union) was published during 1926–1947, the chief editor being Otto Schmidt (until 1941). The second edition of 50 volumes (100,000 entries, plus a supplementary volume) was published in 1950–1958; chief editors: Sergei Vavilov (until 1951) and Boris Vvedensky (until 1969); two index volumes to this edition were published in 1960. The third edition of 1969–1978 contains 30 volumes (100,000 entries, plus an index volume issued in 1981). Volume 24 is in two books, one being a full-sized book about the USSR, all with about 21 million words, and the chief editor being Alexander Prokhorov (since 1969). In the third edition, much attention was paid to the philosophical problems of natural sciences, physical and chemical sciences, and mathematical methods in various branches of knowledge.