Final answer:
Leonin and Perotin were pivotal in the development of polyphonic music, which features multiple independent melodic lines, contrasting with the earlier monophonic style.
Step-by-step explanation:
Leonin and Perotin, composers active in the 12th and early 13th centuries, are known for their work in the development of polyphonic music. This style involves multiple independent melodic lines occurring simultaneously, a significant evolution from the previously predominant monophonic Gregorian chant, which featured a single melodic line. The transition to polyphony marked a pivotal moment in the history of Western music, setting the stage for the complex contrapuntal compositions of the Renaissance and later musical periods.