Harmonic minors use the relative major key signature and raise the seventh note of the scale. For example, in A harmonic minor there are no sharps or flats other than the raised seventh of G# ...

When you use a scale, say, C major, you are playing ‘in the key of C’. The word ‘diatonic’ simply means ‘within a key’, so a diatonic chord progression is a set of chords made up of notes from within ...