The Neapolitan minor is really useful but sadly neglected. In the book The Modal Method, I explain why it should be studied after Major and Melodic but before Harmonic minor. With that in mind I have analysed WTC (The Well Tempered Clavier) book I.
N. minor (abbreviated from now on) occurs in 13 of the preludes (54%) in WTC I and 14 of the fugues (58%). That's how common it's use was.
I am beginning with the second most common way of using N. minor, because believe it or not, the most common relies on the Hungarian minor scale, and as that is not a commonly used scale nowadays, it will be introduced after the first use is explained.
N. minor follows Harmonic minor ten times in WTC I and then resolves to a Major key. For example:
A H. minor - A N. minor - F Major
This then can be summed up as a method of modulating back a fifth from a Major key:
(C Major) - A H minor - A N. minor - F Major
The Major key at the beginning does not have to be used but is useful as a reference guide when writing music, of course starting in the relative minor key is also standard practise.
When in a minor key, using Harmonic minor, flattening the 2nd creates N. minor, in A minor (A Bb C D E F G#). All that is left is for the 7th to be flattened and the new Major key is reached, (A Bb C D E F G) , F Major.
In the example below, I have continued on by changing from F Major to Bb Major but the sequence is the same:
(Major) - H. minor - N. minor - Major
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