It was called Organum
Explanation:
Organum is in a way a precursor to the harmony and it is the first example of polyphony in the medieval chant music wherein one or in some cases until the fourth or fifth a parallel voice was added to the main voice, on the fifth note or sometimes on a parallel note in the same octave.
This parallel voice gave a deeper sense of music and was in a lot of ways the precursor of what would become polyphonic harmony.
