「九」的數學特性

18: 1+8 = 9

27: 2+7 = 9

36: 3+6 = 9

45: 4+5 = 9

54: 5+4 = 9

63: 6+3 = 9

72: 7+2 = 9

81: 8+1 = 9

《易經》把「九」定為陽數，象徵吉祥、光明。

—— 《中國古代文化知識趣談》

"Nine" is used to describe "green object".

Image: amazon.com