Since medieval music was practiced and heard 6 and more centuries ago, it is a very primary kind of music. During this period the theory of harmony was still being developed. That is the reason why many movements which appeared in the music of this period were later prohibited or limited, such as parallel unison, parallel octave, parallel fifth, parallel fourth and pedal point.
The instruments of this period were also very primary and not yet developed. Their timbre is often shrill, narrow and singular, sometimes ugly. Some of them have unstable pitch or false pitch.