Leonin and Perotin are notable because they

There is no specific date or documentation that shows exactly when polyphony started being used in the Church, but two French composers, Leonin and his student Perotin, of the Notre Dame Cathedral in Paris, are generally credited with composing the first significant polyphonic church music.

Explanation:

.                                       sorry i just need points

the answer is counterpoint.

counterpoint definition: "the art or technique of playing a melody or melodies in conjunction with another, according to fixed rules."

counterpoint is present in a lot of baroque music- such as bach (notably his wtc prelude and fugues) and scarlatti, but can also be found in music by mozart, beethoven. it can even be found in music as modern as john williams!