Harmony

Harmony is the use and reading of pitch simultaneity and chords, actual or implied, in music. It is sometimes referred to as the "vertical" aspect of music, with melody being the "horizontal" aspect. Very often, harmony is a effect of counterpoint or polyphony, several melodic lines or motifs being played at once, though harmony may control the counterpoint.

Some traditions of music presentation, composition, and theory have specific rules of harmony. These rules are often held to be based on natural properties such as Pythagorean tuning's low whole number ratios or harmonics and resonances, with the acceptable pitches and harmonies gaining their beauty or simplicity from their closeness to those properties. Other traditions, such as the ban on parallel fifths, were simply matters of taste.