these two intervals have the same number of tones, but different degrees. The number of octaves and Xiao Jiu degrees is six and a half (that is, 6.5), that is, one octave plus one semitone. An octave is an octave, and Xiao Jiu is nine degrees. For example, a rise in C to the next tone group is an octave increase, while a drop in D from C to the next tone group is a Xiao Jiu degree.

the difference between them is just like the difference between increasing one degree and decreasing two degrees. C- up is one degree, and C- down is two degrees. These two intervals are equal intervals in the law of twelve averages, but they have different meanings because of their different degrees, so they should not be confused.