Why an Fdim7 Has a Cb and an Ebb

The subject of music theory tends to raise people’s fur sometimes, but this isn’t a question of right or wrong so much as it is a question of understanding function — and function is what makes harmony GO! Shall I explain it? Do you understand this? I find even experienced musical thinkers get tricked by this one.

In the simplest terms, the four notes in an F major seven chord are based on the first, third, fifth, and 7th degrees of its scale (F A C E). The same is true for an F diminished seven, which is why we name the third as A flat, the fifth as C flat, and the seventh as E double flat. This helps to remind us that this chord is constructed of thirds.