What was the contribution of Leonin and Perotin to organum?

Leonin and Perotin were pivotal figures in the development of early polyphony, specifically through their contributions to organum, a style of music that involved the addition of one or more voices to an existing plainchant melody. The correct answer highlights their significant innovation of creating organum with two vocal parts.

Leonin, often recognized for his work in the late 12th century, is known for composing pieces that feature a principal melodic line, or cantus firmus, which is typically derived from existing Gregorian chant, with one or more additional voices moving in conjunction, thus laying the groundwork for polyphonic texture. Perotin, who followed in the late 12th and early 13th centuries, expanded on this by adding more voices, creating a rich, complex polyphony that defined the music of the Notre Dame school.

Their work not only marked a transition from monophonic to polyphonic music but also established foundational practices in the notation and performance of music, influencing composers in the centuries that followed. Therefore, their contribution to organum can be seen as a critical point in the evolution of Western music, elevating it from simpler textures to more elaborate forms of musical expression.