=The Roundel=, from an historical view-point, was the most important form, for in it much use was made of Imitation. It can best be explained in the words of Walter Odington, a theorist of the time: “Let a melody, with or without a text, in one of the regular modes of rhythm, and as beautiful as possible, be devised, and let each voice sing this in turn. And at the same time let other melodies be devised to accompany it in the second and (if there be three voices) in the third voice; let them proceed in consonances, and so that when one voice ascends another descends, and let the third not follow too closely the movement of either of the others, except perhaps for the sake of greater beauty. And let all of these melodies be sung by each voice in turn.” While the use of Imitation is important in that it recognizes the _repetition_ of a set phrase as an _aid_ to _Unity_, its importance is detracted from, at least at this period, because it was not used in any of the other forms then in vogue. [Music] Roundel for three voices showing Imitation. There are six distinct melodic phrases, and by numbering these wherever they appear, the Imitation can readily be observed. =Imitation a Means of Securing Unity=.—An art form must submit to the laws of the human mind, which demand that a work of art shall show three principles: Unity, Variety or Contrast and Proportion or Symmetry. The problem set before the old composers was to produce musical works which should exhibit obedience to the canons of art as determined in the sister arts which had already reached great perfection. Unity in a musical work means that it is a development of one central thought, in elaborate works, of several leading ideas. The germ of a musical composition is in the Theme. The composer’s problem is to elaborate a piece of some length from this Theme, in that way to secure Unity of idea. If he were limited to writing in one part, he would be compelled to _repeat_ the Theme a number of times, either on the _same_ or on a _different_ degree. When he must write for three or more voices the problem becomes more complicated. Let us imagine a composer of the 12th-century at his work. He has a theme to use, like the one in the example at the end of the preceding paragraph, which he is to use in three parts. From the composers of the preceding centuries he received the principle of _transposing_ the theme a fourth or fifth or octave higher or lower, thus singing the same melody _simultaneously_ at different pitches; but this he rejects as crude; he has passed that stage and wishes to use a newer, more advanced method. Obviously his recourse will be to let each of the other two voices sing the opening theme _successively_ at the same pitch. To stop with this change would result only in three successive repetitions of the opening theme; so he makes the second and third voices sing the phrases used by the first voice after the first theme has been given, which serve as