One person, A, begins this melody alone, and sings it through. When he has reached the third measure, B strikes in at the beginning. When B in his turn has reached the third measure (A being now at the fifth), C comes in in the same way. In a word, the three people sing the same tune _in rotation_ (whence the name, "round"). And the tune, of course, is so contrived that all its different sections, sounded simultaneously by the various voices, merge in harmony. This kind of literal repetition by one part of what another has just done is called "imitation," and is a fundamental principle of all that great department of music known as the "polyphonic," or many-voiced.
But now, notice another kind of repetition in this little tune. Measures 3 and 4 practically repeat, though at a different place in the scale, the three-note motive of measures 1 and 2. (In order to conform to the words, the second note is now divided into two, but this is an unimportant alteration.) The naturalness of this kind of repetition is obvious. Having begun with our motive in one place, it easily occurs to us to go on by repeating it, _in the same voice, but higher or lower in pitch than at first_. The mere fact that it is higher or lower gives it the agreeable novelty we desire, yet it remains perfectly recognizable. We may call this sort of repetition, which, like "imitation," is of the greatest utility to the composer, "transposition," to indicate that the motive is shifted to a new place or pitch.
But suppose we do not wish either to imitate or to transpose our motive, is there any other way in which we can effectively repeat it? Yes:--we can follow its first appearance with something else, entirely different, and after this interval of contrast, come back again and _restate_ our motive just as it was at first. Looking at "Three Blind Mice" again, we see that this device, as well as the other two, is used there. After the fifth, sixth, and seventh measures, which contain the contrast, the eighth measure returns literally to the original motive of three notes, thus rounding out and completing the tune. This third kind of repetition, which may be called "restatement after contrast," or simply "restatement," is also widely in use in all kinds of music. A most familiar instance occurs in "Way Down upon the Suwanee River."