Voices split apart
Two lines at once.
For centuries European church music moved as a single line that everyone sang together. Around 900 CE, Frankish writers set down something new in a treatise called Musica enchiriadis: let a second group sing the same chant shape a fourth or a fifth below, in step with the first. These are the earliest surviving written rules for polyphony in the West.
The choice of distances came straight from the old fractions. Fourths and fifths were the blends singers trusted for a long hold, so the added voice shadowed the chant at exactly those intervals. This practice was called organum.
Play the chant alone, then add each shadow voice. Follow the upper melody while the second line moves with it at a fixed fifth or fourth below. Every note in the added line keeps the same small-number relationship to the chant. Hearing that constant spacing shows how measured intervals became a rule for two simultaneous voices in early Western polyphony.