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159. In our first chapter (§ 10), we defined a Countersubject as "a counterpoint which accompanies answer or subject systematically, though not of necessity invariably." We have now to consider the essentials of a good countersubject, and to endeavour to show when its introduction into a fugue is advisable.

160. The first and most important requisite for a countersubject is, that as it has to accompany either the subject or the answer in whatever part these may appear, it must be written in double counterpoint with the answer, as an accompaniment to which it is first heard. The double counterpoint is, in by far the largest number of cases, in the octave, but it is also sometimes in the tenth or twelfth. Sometimes a countersubject which has been used in the octave at first is subsequently employed in one of the other intervals. In Double Counterpoint will be seen in § 163 an example of a countersubject used both in the octave and in the tenth; and at § 175 of the same book a similar instance of the octave and the twelfth.

161. A most important point in writing a countersubject is individuality of melodic character, and contrast of rhythm as compared with the subject (Double Counterpoint, § 129). This will be best illustrated by examples, which we shall mostly take from Bach's 'Wohltemperirtes Clavier,' as being a work accessible to everybody.

162. In our first examples