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Harmony is the musical effect observed when two or more pitches interact in the same perceived time element.
Harmony looks at the sum of all notes and how those notes interact with each other. The main branch of studying harmony is called diatonic harmony, which is based off of concepts from the common practice period.
Harmony can exist to varying degrees of complexity, going far beyond simple diatonic harmony all the way to abstract chromaticism.
Chords - groups of three or more notes sounded simultaneously - are a useful concept to study harmony, and the relationship between chords can be analyzed to study the ebb and flow of music. Musicians often analyze using structural concepts like chords even if the original composer predates the ideas used to study their works.