According to inyo-shiso (a Chinese ideology of dual cosmic forces, the active and passive), odd numbers are regarded as active numbers, among which the number nine is the ultimate, so that dual nine (September 9) means the highest level of activeness and is written in Chinese characters as "cho" (meaning "doubled") and "yo" (meaning "activeness").
陰陽思想では奇数は陽の数であり、陽数の極である9が重なる日であることから「重陽」と呼ばれる。