This is related to the note by Jo Gen in "Yi wei", the surviving fragment of a book on Shinisetsu (isho) which was banned by Emperor Yodai of Sui and got scattered and lost, where 60 years, one cycle of the Oriental zodiac, were defined as 1 gen and 21gen as 1 bou, and it was suggested that, in a specified zodiac year every 1260 years (= 60 ' 21) thus calculated, a national-scale revolution (change of dynasty) would be carried out.
これは隋の煬帝により禁圧されて散逸した讖緯説の書（緯書）の逸文である『易緯』の鄭玄の注に、干支が一周する60年を1元（げん）といい、21元を1蔀（ぼう）として算出される1260年（＝60×21）の辛酉（しんゆう）年に、国家的革命（王朝交代）が行われる（辛酉革命）という事に因む。