Rule 2. The Fourth of a Scale is the Tonic of the Scale having the Next Number of Flats.
C has no flats; the fourth of C is f; therefore, by following the rule, we find that F has one flat:—
F g a bb c d e F
1 2 3∼4 5 6 7∼ 8
Notice the fourth of the scale is a black key.
The fourth of F is bb and has two flats:—
Bb c d eb f g a Bb
1 2 3∼4 5 6 7∼8
Notice that the b remains flat and that the added flat is the fourth of the scale. This is always the case—the added flat is the fourth of the new scale.
The fourth of Bb is eb and has three flats:—
Eb f g ab bb c d Eb
1 2 3∼4 5 6 7∼ 8
The fourth of Eb is ab and has four flats:—