ee conditions are necessary to
make a canon into a round. First, the voices must imitate each other in
the unison; secondly, they must enter at equal intervals of time; and
thirdly, the whole melodic material must be as many times longer than
the interval of time as the number of voices; otherwise, when the last
voice has finished the first phrase, the first voice will not be ready
to return to the beginning. Strict canon is, however, possible under
innumerable other conditions, and even a round is possible with some of
the voices at the interval of an octave, as is of course inevitable in
writing for unequal voices. And in a round for unequal voices there is
obviously a new means of effect in the fact that, as the melody rotates,
its different parts change their pitch in relation to each other. The
art by which this is possible without incorrectness is that of double,
triple and multiple counterpoint (see COUNTERPOINT). Its difficulty is
variable, and with an instrumental accompaniment there is none. In
fugues, multiple counterpoint is one of the normal resources of music;
and few devices are more self-explanatory to the ear than the process by
which the subject and counter-subjects of a fugue change their
positions, revealing fresh melodic and acoustic aspects of identical
harmonic structure at every turn. This, however, is rendered possible
and interesting by the fact that the passages in such counterpoint are
separated by episodes and are free to appear in different keys. Many
fugues of Bach are written throughout in multiple counterpoint; but the
possibility of this, even to composers such as Bach and Mozart, to whom
difficulties seem unknown, depends upon the freedom of the musical
design which allows the composer to select the most effective
permutations and combinations of his counterpoint, and also to put them
into whatever key he chooses. An unaccompanied round for unequal voices
would bring about the permutations and combinations in a mechanical
order; and unless the melody were restricted to a compass common to
soprano and alto each alternate revolution would carry it beyond the
bounds of one or the other group of voices. The technical difficulties
of such a problem are destructive to artistic invention. But they do not
appear in the above-mentioned operatic rounds, though these are for
unequal voices, because here the length of the initial melody is so
great that the composition is quite long enough before th