The development of harmony, the ratios of vibration that introduce degrees of dissonance into music, evolved out of the organum of the late 10th and 11th centuries. While credit for this development is unquestionably shared by many composers of the age, let’s focus on two.
Hildegard of Bingen was by any measure a remarkable woman. A famous mystic of her age, this 12th century Benedictine abbess was a prolific writer, composer, philosopher, and scientist. In addition to her theological and scientific texts, Hildegard’s musical legacy is legion—there are more surviving chants by her than any other composer of her age. In 2012, Pope Benedict XVI declared her to be a saint and a doctor of the Church.
Hildegard’s music is timeless, with a sense of otherworldliness that comes from her use of the drone and the Phrygian mode. A “mode” is one of seven series of notes that were developed by the Church during the Middle Ages. For example, the “Ionian” mode can be replicated by playing a C major scale starting on C (C – D – E – F – G – A – B). The “Phrygian” mode is the same C major scale, but beginning on E. A fuller description of the various modes can be found here: https://www.classicfm.com/discover-music/latest/guide-to-musical-modes/, but will note that the Phrygian scale is among the most evocative emotionally, as it is a type of minor scale, with minor seconds, thirds, sixes, and sevenths to complement the perfect fourths and fifths. If you play just the white keys on a piano starting with E, that’s the Phrygian scale.
Hildegard’s O virtus sapiente is among my favorite works of music. The Phrygian melody is written above an E drone, which clashes in spectacular dissonance with the F, before immediately resolving.
Hildegard of Bingen, O virtus sapiente:
Harmony took a big step the following century at the new cathedral of Notre Dame in Paris. Here, a 13th century composer named Perotin came up with the radical notion that you could have 2, if not 3 or 4 voices singing together. He also experimented with very rudimentary chords to his music, revolving around so-called “perfect” 4ths and 5ths. Perotin was also responsible for adding rhythmic symbols to musical notation—allowing performers to know not just what note to sing, but also how long to sustain it.
Perotin, Viderunt omens:
These early composers continue to influence the development of music today. For example, consider Grace Slick’s use of the Phrygian mode in this classic tune, which is just as eerie as Hildegard’s creations some 900 years earlier:
Jefferson Airplane, White Rabbit:
A Gertus History of Music 