Harmonic structures refer to the way chords and notes are organized in music to create a sense of progression and resolution. They form the foundation of a piece, influencing its emotional character and overall sound. Common harmonic structures include major and minor scales, which serve as the basis for many compositions. In Western music, harmonic structures often follow specific patterns, such as the I-IV-V-I progression, where chords are built on the first, fourth, and fifth degrees of a scale. These structures help musicians convey feelings and ideas, making them essential for both composition and performance.