Counterpoint is to-day the art of constructing two or more melodies which can be sung or played simultaneously without breaking the rules of harmony. Originally, however, it was the art of adding parts above or below a part already selected. It originated, as we have seen, in the practice of the descanters. A part improvised by a descanter came to be called _contrapunctus a mente_ (a counterpoint out of the head), while an additional part written by a composer was called _contrapunctus a penna_ (a counterpoint from the pen). As musicians acquired skill in the construction of these additional parts, they began to introduce new devices, and to write with greater and greater freedom. The more free their writing was, the further it tended to depart from parallel motion. In the course of time some one hit upon the musical device called "imitation," which means the repetition in a secondary part (say the bass) of some passage already heard in the principal part (the treble) while that principal part is still going on. The result of this device is that one portion of a melody is made to serve as the second voice to another portion of it. Who first hit upon this device, no one knows; but the earliest example of it which has been preserved is found in the "Posui adjutorum" of Perotin, one of the first of the French school of writers. Here is the passage; and by giving it careful study the reader will be able to understand the fundamental principle of canon, fugue, and all the polyphonic forms:-- [Music: 1st Voice, 2nd Voice, Canon Form.] I think that even a reader who is not a musician can understand this. A is the first half of a melody, and B is the second half. While the first voice is singing the first half, the second voice sings the second half as the alto part of the first half. In order to make the first half act as alto in the second half, the composer had to push the second voice one bar ahead of the first voice, and then to add three extra notes to A in order to make a conclusion to his alto part. There are two imitations in this bit: the second half of the part sung by the first voice imitates the first half of the second voice part, and the second half of the second voice part imitates the first half of the first voice part.