Counterpoint is today 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:—

[Listen: "Posui adjutorum" of Perotin]

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[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.

By extending and developing such imitation as this, composers came to write in "double counterpoint," which means the construction of two parts in such a way that their different portions can be transferred from one to the other just as they are in the selection from Perotin. The reader will see at once that composing in this manner required a great deal of calculation, and was a constant tax on the ingenuity of the musician. In its early stages it prevented any attempt at making music expressive, and reduced composition to a mere exercise of scientific skill. But it forced the composers to a close study of the materials of their art, and they acquired a great mastery over them and constantly learned more and more about the possibilities of music. Double counterpoint, which is at the foundation of the most rigid forms of polyphonic writing, was generally known in the French school at least as early as the thirteenth century, but the example from Perotin seems to show that at least some of the composers knew it much earlier. This might account for the existence of the celebrated example of early English polyphony, a "canon" called "Sumer is icumen in." This was discovered by Sir John Hawkins, who wrote a history of music in 1776. Its manuscript was copied by a monk of Reading, John of Forneste, in 1228, and it must have been composed shortly before that time. The fact that Walter Odington, an Englishman, wrote a treatise on music in 1230, when the only famous school was that of Paris, leads me to believe that these early English composers were disciples of the French. But it is certain that the composer of "Sumer is icumen in" was a greater master of counterpoint than his teachers, because this "canon" is the finest specimen of polyphonic writing that has come down to us from those early times. Here is its beginning:—

[Listen: "Sumer Is Icumen In"]