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. Every note of the added line keeps one fraction against the chant, which turned a measured interval into a rule for two voices singing at once. This practice was called organum.
Play the chant alone, then add each shadow voice. Listen for the second line to hold a fixed fifth or fourth below the melody all the way through.