42. A good fugue subject should always (or at least with extremely rare exceptions) contain a complete musical phrase. By the word "phrase" is here meant a passage containing some distinct idea, and terminating with a cadence of some kind (Counterpoint, Chapter XV.). It is not intended by this that there is actually to be a cadence introduced at the end of the subject, but only that the final notes of the subject shall be capable of being harmonized as a cadence—not necessarily a full cadence, though in a large majority of instances this is the case. If the student will examine all the subjects already quoted in this chapter, he will find this condition invariably fulfilled. In almost every instance the subject ends with either the root or third of tonic or dominant. In the rare cases where the subject ends on the subdominant (see § 38, (c), (d), the close will be made on the root or third of the tonic of that key.
43. In order to obtain a proper cadential effect, it is necessary in common time that the subject should end on an accented note—either at the first or third beat of the bar (see for instance the examples in § 28). In this case, if the cadence is felt as occurring on the strong beat in the bar, a continuation of the harmony is sometimes added, as in § 31 (c).
44. The only exception to this rule in common time is, that the subject may end on an unaccented note, provided that the preceding accented note has the character of a suspension or an appoggiatura—
Here the end of the third bar suggests the chord of the dominant seventh; it is resolved at the beginning of the fourth bar, where the B is clearly an appoggiatura. See also examples § 32 (b) and § 35 (b).
45. In triple time this rule does not apply, because here it is possible for the final chord of the cadence to come on the second beat (Counterpoint, § 484). As an example of this see § 34 (d).
46. It is impossible to give any definite rules as to the length of a fugue subject. In the works of the great masters we sometimes find them quite short, consisting in fact of only a few notes—
Wohltemperirtes Clavier, Fugue 5.
Wohltemperirtes Clavier, Fugue 22.
