Organon, which originated in the 9th century and is dominated by parallel voices, is the earliest form of polyphonic music.

Subsequently, church music forms such as Clausura, motet, and Konductus emerged one after another. The increase in forms will inevitably lead to the enrichment of creative techniques. First of all, the arrangement of voices has developed from the original two voices to three or four voices.

Parallel parts refer to the two parts of the music proceeding in parallel, separated by a perfect fifth. Since the fifteenth century, counterpoint and harmony have stipulated that parallel voices should be avoided.