To dissect out the causes and effects of this tendency would be a complicated and difficult task for which there is no space here. But there is one point which is a matter of common knowledge to the trained musician, and the general musical public is probably more or less aware of it though unable to explain it in technical language. From the year 1600 to the year 1900, roughly speaking, all Western music is based on the same fundamental principle of _tonality_. All music is composed in a key. One note is adopted as a centre. The remaining notes of the octave are brought into various clearly defined relationships to it. They may further be arranged in groups, sounded simultaneously, known as chords. Each of these chords has its own fixed arrangement and its fixed relationship to the centre. What has been done for one note of the octave may be done in exactly the same way for any other, forming what we call the key of that note. The musician may shift from one key to another in the course of his work, but it is understood that he must make his main key clear and definite at the outset and must re-establish it again with equal decision at the end. In the early years of the seventeenth century the efforts of musicians were directed chiefly to establishing one key clearly and towards the training of audiences to grasp the first principles of the system. As they became more and more accustomed to the system the composers were able to extend and elaborate it. The interrelations of notes and chords became increasingly subtle and delicate from the days of Monteverdi to those of Wagner; but the fundamental key-system and the rhythmical system which is inseparable from it remained always precisely the same. The language of music developed steadily and rationally just as the English language has developed from Shakespeare to Swinburne. It is no wonder then that most musicians regarded its foundations as indestructible.