The organum, which flourished at Notre-Dame Cathedral in Paris, was one of the first types of polyphony. It was very similar to a trope, as it added vertical notes to an existing melody or plainsong. We note a certain development of the organum between the 10th and 12th centuries. The French composers Léonin and Perotin were the main contributors to the evolution of the organum by putting forward the terms "organum libre" and "organum discant". Examining the works of Musica enchiriadis from the 10th century and the compositions of Leoninus and Perotinusis in the 12th century, it becomes clear that the organ underwent influential alterations both melodically and rhythmically. The 10th century organum presents a fourth, a fifth duplicate. , or octave below the plainsong melody in the lead voice. The duplicated voice moves in an oblique and/or contrary movement, with a multitude of interspersed intervals. In addition to oblique and contrary movements, there are parallel and similar movements. All of these types of movements can be present in an organum, but by the 10th century, oblique and contrary movements were the most typical. An example of an organum from the 10th century can be found in Musica enchiriadis. Musica enchiriadis is an anonymous treatise which was the first of its kind to describe polyphony. It established a system of guidelines for polyphony and included many examples of organum. While the examples in Musica enchiriadis are mere teachings and instructions on how to sing an organum, some of the earliest examples of organums ever recorded exist in the treatise. As the 12th century approached, the organum resisted many modifications in terms of rhythm and melody. In addition, the terms "free or...... middle of paper ......uadruplum", in four votes. Even though more voices were added, there was still a constant hum that kept the melody of the singing underneath. These changes in the organum are presented in Perotinus's Viderunt omnes. In addition to the added voices, there are rhythm modes in the upper voices that repeat phrases. Using Musica enchiriadis as an example from the 10th century and the works of Leoninus and Perotinusis as examples in the 11th and 12th centuries, it is evident that the organum underwent many changes between the 10th and 12th centuries, both melodically and rhythmically, including including adding voices, changing movement and developing rhythm. These adaptations of the organum, although they may seem insignificant, have contributed enormously to the evolution of polyphony in Western music, which thus contributes to the music of today..