Canon
Imitation of a complete subject by one or more voices
at fixed intervals of pitch and time. If each successive following
voice ("comes") follows the leading voice
("dux") in every detail, the canon is strict; if, however, the "comes"
modifies the "dux" by minor changes in accidentals, the canon is free.
Canons may be self-contained entities or may occur within larger pieces
(canonic imitation). They may also be combined with independent lines
(mixed or accompanied canons) or even with other canons (groups or compound
canons).
Canons are usually classified on the basis of the following elements:
1. The time between entries -- "canon ad minimam" (at
the half note), "ad semibrevem" (at the whole note), etc.
2. The interval between entries -- "canon ad unisonum"
(at the unison), "ad epidiapente" (at the fifth above)
3. Transformations of the subject, as follows:
a. Inversion, in which the "comes" imitates
the "dux" upside down
b. Retrograde, in which the "comes" gives
the "dux" backwards
c. Retrograde Inversion, in which the "comes"
gives the "dux" upside down and backwards
d. Augmentation,
in which the note-values of the "comes" are longer by a fixed ratio
e. Diminution,
in which the note-values of the "comes" are shorter
f. A mensuration canon is one in which the
"dux" is interpreted simultaneously in different mensurations or proportions,
with the result that the temporal relationships between the voices may
shift because of the different interpretation of individual note-values.
4. Ending - Finite canons have
a definite ending that may either add notes to the "dux" to make up the
time lag between the first and last entries or perhaps add a short coda;
infinite canons (perpetual canon, circular canon,
round, "rota") lead straight back to the beginning with an arbitrary
ending shown by a fermata. In the case of the modulating canon (spiral
canon), the "dux" ends in a key different from the one in which
it begins; a specific case is the "canon per tonos," in which the "dux"
ends in a key a whole-tone higher than the key in which it begins, returning
to the original key only after six statements, as in Bach's
"Musical Offering" (BWV 1079).
5. Number of canons - Canons combined with other canons
(group or compound canons) are indicated by the number of canons (double
canon has two; triple canon has three; etc.) and by the number of
parts. A two-part double canon has four parts and is thus a canon
"four-in-two."
The technique of canonic imitation, that is, of canon in the present-day
sense, was first used in the 13th century. Several motets
include long passages in canonic imitation. Also in this category
are the caccia and chace
of the 14th century. A number of 14th- and 15th-century motets
and Mass compositions open with a section in canonic
imitation. Prominent in late 15th- and early 16th-century music (Josquin
and others) is the canonic chanson, in which
two voices form a canon. Palestrina
often used canons in the last Agnus Dei of his Masses. It was chiefly
through Bach that the canon won
an important position in music, a position it has maintained to the present
day.