Why in Music Do We Have Major and Minor Scales/Key Signatures but No Separate Key Signatures for Harmonic or Melodic Minor Scales?

When delving into the world of music theory, one might naturally wonder why there are no separate key signatures for the harmonic and melodic minor scales, especially given their widespread use. Let's explore this fascinating aspect of music theory and how it fits into the broader context of musical composition.

Understanding Key Signatures

At the core, a key signature serves a single, crucial function: it tells you how many sharps or flats are to be used in a piece of music. There are no separate key signatures dedicated to major and minor scales alone. For instance, if a key signature is completely empty, like in the case of C major and A natural minor, it doesn't instantly reveal whether you're dealing with a major or minor scale. This ambiguity means that you need to look at the first and last notes, as well as the modulation points, to determine the specific scale used.

During one's college years, students often face the task of writing simple tunes. In a recent scenario, an author chose E minor, correctly placing F in the key signature. However, the recipient of the tune incorrectly assumed it was G major because of the single F in the key signature. This example highlights the ambiguity and the importance of context in determining scale and modulations.

Key Signatures and Modes

Major and minor scales can be seen as modes of each other; they are simply different arrangements of the same set of notes. The other five modes—Dorian, Phrygian, Lydian, Mixolydian, and Aeolian—are also considered "relatives" but are rarely utilized in common practice. The key signature dictates the sharps or flats, providing a consistent reference for the notes to be used in a piece of music.

In essence, the key signature represents the natural mode, altered modes like harmonic minor, and synthetic modes like whole tone scales and octatonic scales. The harmonic and melodic minor scales, while fitting within their given key signatures, have specific alterations to cater to the harmonic needs of the composition. This includes the raised leading tone of the V chord and avoiding the augmented second between the flatted sixth and seventh degrees in a melody.

Historical Context and Practicality

Historically, it was common to use the key signature of the dominant key for shorthand purposes. For example, Baroque compositions in G minor might use D minor as the key signature due to the frequent use of E natural. This practice highlights the flexibility of key signatures and how they can be employed to simplify complex compositions.

Some "exotic" modes, like those found in Klezmer and Balkan music, might use the key signature of the subdominant key to create a unique sound. For instance, a song in A Mixolydian might use the key signature of D, aligning with the natural notes of A Mixolydian. Similarly, a song in D minor can have a Bb, Eb, and F in the key signature, technically G harmonic minor, due to the tonic being D, a form known as "Spanish Phrygian" on the guitar.

This complexity underscores the importance of understanding harmonic and melodic minor scales, as they provide a significant advantage in composition and performance. Practicing these scales not only enriches your musical vocabulary but also enhances your ability to navigate complex chord progressions and melodic lines.

So, while there are no separate key signatures for harmonic and melodic minor scales, their frequent use and the necessity of understanding their altered notes make them essential components of a musician's toolkit. Embracing these scales and their roles within the key signatures enhances the depth and expressiveness of your musical arrangements.