a short movement
on one idea indistinguishable in form from a _fughetta_ of Bach; as in
the _Kyrie_ of Palestrina's Mass, _Salve Regina_.

But in Bach's art the preservation of a main theme is more necessary the
longer the composition; and Bach has an incalculable number of methods
of giving his fugues a symmetry of form and balance of climax so subtle
and perfect that we are apt to forget that the only technical rules of a
fugue are those which refer to its texture. In the _Kunst der Fuge_ Bach
has shown with the utmost clearness how in his opinion the various types
of fugue may be classified. That extraordinary work is a series of
fugues, all on the same subject. The earlier fugues show how an artistic
design may be made by simply passing the subject from one voice to
another in orderly succession (in the first example without any change
of key except from tonic to dominant). The next stage of organization is
that in which the subject is combined with inversions, augmentations and
diminutions of itself. Fugues of this kind can be conveniently called
stretto-fugues.[2] The third and highest stage is that in which the
fugue combines its subject with contrasted counter-subjects, and thus
depends upon the resources of double, triple and quadruple counterpoint.
But of the art by which the episodes are contrasted, connected climaxes
attained, and keys and subtle rhythmic proportions so balanced as to
give the true fugue-forms a beauty and stability second only to those of
the true sonata forms, Bach's classification gives us no direct hint. A
comparison of the fugues in the _Kunst der Fuge_ with those elsewhere in
his works reveals a necessary relation between the nature of the
fugue-subject and the type of fugue. In _Kunst der Fuge_ Bach has
obvious didactic reasons for taking the same subject throughout; and, as
he wishes to show the extremes of technical possibility, that subject
must necessarily be plastic rather than characteristic. Elsewhere Bach
prefers very lively or highly characteristic themes as subjects for the
simplest kind of instrumental fugue. On the other hand, there comes a
point when the mechanical strictness of treatment crowds out the proper
development of musical ideas; and the 7th fugue (which is one solid mass
of stretto in augmentation, diminution and inversion) and the 12th and
13th (which are invertible bodily) are academic exercises outside the
range of free artistic work. On the other hand, the l