scale degree

A numeric musical interval is determined by counting inclusively between letters. For instance, when counting as shown:

    F   G   A   Bb
    1   2   3   4

... you'll find that Bb is a 4th from F.

B notes are flat in the key of F so the interval is simply a 4th—called a Perfect 4th, i.e. the 4th degree of the F major scale, a distance of 5 semitones. (Count semitones omitting the first letter:

     F   F#    G   G#  A   Bb
           1     2    3     4    5

It's easy to think that a flat makes an interval minor, because sometimes it does.

Here are a few grammatical rules that govern the use of the words "minor" and "diminished."

If unaltered from their respective positions in a major scale:

    • 1sts, 4ths, 5ths and 8ths (octaves) are Perfect intervals

    • 2nds, 3rds, 6ths and 7ths are Major intervals

    • When Major intervals are flatted they are call minor.

    • When Perfect intervals are flatted they are called diminished. (Logically a b4 is the same as a "minor 4th" but the term "minor" isn't used to describe a b4.)

Here's an example:

The 4th degree of a D major scale is A. The flatting that note results in an Ab which is a diminished 4th from D.

It takes most people a bit of study to get comfortable with music core grammatical rules. So hang in there.

scorster

There is no such thing and a minor fourth. The interval from the first note of a major scale to the fourth note is called a perfect fourth. Just as the interval from the first to fifth note is called a perfect fifth. The intervals from the first to second, third, sixth, and seventh notes are called major. That's just how it is; for reasons that are hard to explain but not really all that necessary to understand.