Why do we start from the bottom note when counting intervals?

Question

Say if I play G to C descending, my intuition tells me to start from the note that came first. But I've been told that's not correct. You start from the bass note and count up no matter what.

Answer 1

I think it's important to be specific, especially considering the direction of the interval.

For example, G is a 5th ABOVE C, and a 4th BELOW the next C. The operative word in your first sentence was "descending," so that's why "a 5th" is the answer here.

Answer 2

Intervals can be described in either direction depending on context. While chords are described from bottom up, when identifying an interval on it's own, either direction is fine.