Ten experts decide who and what changed the course of music history.

In the 12th century, two extraordinary composers were experimenting in their compositions. The German abbess Hildegard von Bingen was an unusual individual – feminist, painter, gardener, herbologist – a polymath of some distinction. She wrote mystical poems, which she set to music for her nuns to sing, such as Columba Aspexit and Ave Generosa. Her compositions were longer, greater in range and more ornate than Gregorian chant and moved out of the modal system by introducing unusual chromaticisms that, even today, require the expertise of professional singers. She was an early composer of liturgical dramas, of which Ordo Virtutum is the best known.

Around the same time in Paris, the French composer Pérotin, based at Notre Dame, was doing something very different. As well as creating short, multi-voiced pieces, he wrote massive organum triplum and organum quadruplum. Works like Sederunt Principes sometimes lasted up to half an hour. This new, multi-voiced music in three and four parts was based on Gregorian chant, using the chant melody as the fundament. Above it, Pérotin composed two or three swifter moving parts in measured rhythms.

Winsome Evans is Honorary Associate...