The term "fuga" was used from the late Middle Ages to the early
Baroque for strict imitation or canon,
but fugal writing in the modern sense first appears in 16th-century vocal
polyphony and in instrumental forms, including the ricercare,
and canzone, derived from it.
Fuga, in its present sense, appears alongside 'fantasia' in the "Tabulature
nova" (1624) of Scheidt. Another kind of fugue emerged from the keyboard
of Froberger and Buxtehude, and
the idea of including fugal passages in the toccata
to the 'prelude and fugue' combination. J.C.F. Fischer's "Ariadne musica"
(1702) is a collection of preludes and fugues in various keys which served
as an example for Bach's "Well-tempered Clavier".
Construction and Processes
A typical Bach fugue would be constructed as follows.
In the opening section, the exposition,
the main theme or subject is announced
in the tonic. After which the second voice
enters with the answer, i.e. the same
theme, usually at the dominant pitch, while
the first may proceed to a countersubject.
The answer may be either a "real" answer which is "in" the dominant key
(having all the pitches of the subject transposed to the new key), or a
"tonal" answer, which is "on" the dominant pitch level but where some pitches
are slightly altered to correspond more to the original, tonic key.
This procedure is repeated at different octaves until all the voices have
entered and the exposition is complete. An extra statement of the subject
or answer following the exposition is called a 'redundant
entry'; a set of such entries is a 'counter-exposition'.
The exposition is the only essential for the definition of
a piece as a fugue, but most fugues proceed to further entries of the subject,
which may be separated by 'episodes',
often based on material from the exposition, often tonally unstable and
modulatory, frequently employing the use of sequences. The 'middle
entries', normally in keys other than the
tonic or dominant, may treat the theme in stretto
(with overlapping entries) or vary it in some way. In 'augmentation'
the note values are lengthened, in 'diminution'
they are shortened; in 'inversion' the subject
is upside down, in retrograde the subject
appears in reverse note order. A 'false
entry' begins the subject but does not complete it. The final
entry of the subject is usually in the tonic
key. Often a closing section
appears, utilizing a pedal point on the dominant pitch and/or emphasizing
the subdominant key.
Bach's "Well-tempered Clavier,"
in two volumes, contains some of his greatest fugues
but did not exhaust his command of fugal technique. His
and cantatas frequently combine
fugue and ritornello form,
and he introduced the combination of the subject and a number of counter-subjects
in various vertical permutations. In the "Art
of Fugue" he explored the potentialities of a single main
theme in a cycle of 14 fugues, including pairs of
invertible or mirror fugues, a species unique to this work.
The Toccata in D minor
is an example of the form established by Buxtehude, in which the fugue
is interspersed with sections of free fantasia. The Passacaglia
in C minor serves as a prelude to a double fugue, one of whose
subjects is identical with the first half of the passacaglia theme.
From the later years of Bach's life comes the gigantic Prelude
in E-flat major, and the Fugue ("St. Anne's") in the same key.