What Is the Sixth Cord?

A perfect sixth? That doesn't exist. No, because we found this interval directly in the overton series and it's not perfect, so it's major. There's no other options. Counting to 15, we'll find the seven different knots of what we call the major scale. But the nots will, of course, be spread out over various octaves, with some notes repeated. So we scale these knots down to fit them all into a neat octave - that's where the major scale comes from.