How do harmonic minor and melodic minor scales alter the natural minor scale?

In Western music, the natural minor scale serves as the foundation for the harmonic minor and melodic minor scales. These variations are primarily used to address the lack of a leading tone—a crucial element in diatonic harmony—found in the natural minor scale.

• Harmonic Minor Scale: This scale alters the natural minor scale by raising the seventh tone by a half step. This modification creates a leading tone, which is essential for constructing harmony and establishing a strong sense of resolution toward the tonic.12
• Melodic Minor Scale: This scale features different intervals depending on the direction of the melody. When ascending, it raises both the sixth and seventh tones of the natural minor scale by a half step, which helps smooth the melodic line. When descending, the melodic minor scale reverts to the natural minor scale, using the original lowered sixth and seventh tones.12

While the natural minor scale defines the key signature, these chromatic alterations are typically indicated by accidentals within the musical notation.2