The harmonic minor scale is also commonplace in classical ... which is where this scale really shows its purpose. An F note (the b5) is the target choice for the Bm7b5. All these note choices ...

This creates a mode of A Harmonic Minor which is called E Phrygian dominant ... having already used the b9 (F) for a more moody colour. This is over the contrasting harmony on the IV chord ...

Harmonic minor keys are used when writing chords. There is another version of the minor key known as the melodic minor. This is used when writing a melody. Each key signature has two possible keys.