The other thing to keep in mind is that in musical intervals, X and Y add up to X + Y - 1. For example, two thirds (say, C-E and E-G) make a fifth (C-G). That

The other thing to keep in mind is that in musical intervals, X and Y add up to X + Y - 1. For example, two thirds (say, C-E and E-G) make a fifth (C-G). That's because both endpoints get counted in the interval.