Recall from Chapter 15 that a major scale whose tonic lies a perfect fifth below the keynote of C requires the addition of a flat in the key signature (F major). Go down a further fifth to Bb, and another flat is required. This pattern can also be further extended, as shown in Figure 19.2.
Each key whose tonic is a fifth lower requires a further flat in the key signature.
In this way another set of keys are generated—those keys with flats. The further on in the scheme of falling fifths, the more flats the key has. It is interesting to observe that ...