How can a single melody suggest underlying chords and a key?

In Western music, a single melody can imply underlying chords and a key because the human ear is capable of perceiving harmonic patterns even when notes are sounded successively rather than simultaneously. This phenomenon relies on the listener's ability to recognize the "inner logic" of a melodic line, which often functions as the surface of a harmonic structure.12

Composers achieve this implication through several techniques:

• Arpeggiated Chords: By playing the notes of a triad or other chord structures in succession, a melody clearly outlines the harmonic foundation.1
• Scale Passages: Frequent use of specific scale degrees helps establish the tonality and key of a composition.1
• Melodic Contour: The overall shape of a melody—including its leaps, steps, and points of tension—can indicate harmonic movement. For instance, certain melodic intervals or patterns naturally suggest a resolution to a more stable harmony.123

By utilizing these elements, a melody provides enough context for a listener or musician to deduce the intended harmonic progression and key, even in the absence of explicit accompaniment.12