A chord may be defined as a combination of three or more different tones conceived as a related unit and sounding at the same moment in time or arpeggiated. An arpeggio is a rapid alternation of chord tones each occurring with one following the other in time. Although harmony is non-existent in many world musics, it is found extensively in the music of the West where it has gained complexity with the passage of time.
Many world cultures employ only melody in their music.
Western music began as a melodic music only. It was not until the Renaissance period that harmony was introduced as a basic element in the music of the West. As a basic element of music, harmony continued to evolve in complexity until the beginning of the 20th century in the West. Chords exist in a scale-tonal infrastructure. Certain chords project a quality of instability and tension and are known as dissonant chords while other chords project a quality of stability and repose and are known as consonant chords .