For minor keys, there are no closely related keys built on the second scale degree. Why? Is it because the chord built from the second scale degree is diminished?
Closely related keys are keys with key signatures within one accidental of each other. Find the number of accidentals in the given key signature, then find the keys that have that same number (and type) of accidentals, one less, and one more.
For instance, C minor has three flats in its key signature. The closely related keys to C minor will be:
- E♭ major (also three flats)
- B♭ major (two flats)
- G minor (two flats)
- A♭ major (four flats)
- F minor (four flats)
Another way to go about determining the closely related keys is to think through the diatonic Roman numerals of the key. In a minor key, those chords are i, ii°, III, iv, v, VI, and VII. When we translate these Roman numerals to keys, we get:
- C minor
- We skip ii° (D diminished) because it's not a key (at least, not in the Classical style).
- E♭ major
- F minor
- G minor
- A♭ major
- B♭ major
Taking the minor to be a minor third from its relative major. Thus the ii° of Cm is the same as the vii° of E♭ major. That is called D°, and spelled D, F, A♭, using the same scale notes as E♭ major - C natural minor. If C melodic minor scale notes are used, that ii chord becomes Dm, spelled D, F, A.
The other question here concerns 'closely related' keys. Adding or taking away one ♯ or ♭ from the key signature. gives two options. Going to the parallel chord is another.
This question is now confusing to answer: of course there is a chord made on the second degree of Cm. It's D diminished, which is actually a chord, but not one that denotes (sic) a good resting place. rather it's dissonant, which is actually good, because it can lead to several other chords quite happily.
In standard usage, we determine the closely related keys through the diatonic chords' Roman numerals of the key. A closely related key is any key whose tonic triad is diatonic to the original key, and does not exceed one sharp or flat in its key signature.
In natural minor, the chords are known to be as i, ii°, III, iv, v, VI, and VII. The diatonic chords of C minor are Cm, Ddim, E♭, Fm, Gm, A♭, and B♭. When we translate them to keys in the case of C minor, we get:
- C minor (i, three flats)
- We skip Ddim (ii°) because it is not a key.
- E♭ major (III, also three flats)
- F minor (iv, four flats)
- G minor (v, two flats)
- A♭ major (VI, also four flats)
- B♭ major (VII, also two flats)