In the high medieval period, Europe began to move away from the simple monophony that characterized most world music. The first development in this direction was a style known as organum.
In the simplest form of organum, a melody is sung identically, but on pitches a fifth apart from each other. This is called parallel organum.
The next development took one of these voices and held it on a single pitch. This is called drone organum. Although this makes the second part simpler in a strict sene, drone organum is more sophisticated from a music theory perspective, because it introduces a variety of different intervals into the music. This was the first foray into harmony, the distinguishing element of European music.
Intervals are characterized based on how far apart the notes lie on the staff. Two notes on the same line are called unison (from the Latin word "uno," or "one.") A note on one line and another note on the adjacent space are a second apart. Notes with a line or space between them are a third away from each other. Next comes the fourth, fifth, sixth, seventh, and finally you will reach the double-frequency octave (from the Latin word for "eight," since the notes are eight scale-tones away from each other.)
The most advanced type of organum is the first example of true polyphony, with multiple melodic lines moving independently of each other. Organum with two melodic lines is called organum duplum; three lines are organum triplum; four lines make organum quadruplum.
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