Quote:
Originally Posted by nojtje
On a quick note, another fugue - which is very famous, actually - which uses the sequence of tonic - subdominant -tonic etc. in the exposition is the fugue in Bach's G Minor Sonata for Solo Violin. While it's a fugue in a totally different league (after all, true polyphony on a violin is pretty much impossible) it is an example of a subject that strictly requires being answered in the subdominant. But even in the Well-Tempered Clavier, there are no fugues of this model, so the G Minor Sonata is somewhat an odd exception.
Furthermore, the question of why the sequence tonic - dominant - tonic actually establishes the tonic better than other sequences is because from a harmonic point of view, the dominant is the only scale degree leading to the tonic, and thereby creates a 'cadence' in the tonic. A perfect cadence is much more firm than a plagal one, as is easily heard by trying both out on a piano. Therefore, while I'm sure other sequences can work, they were not preferred by former fugue composers as tonality and harmony always came before polyphony, and the key had to be firmly established somehow.
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Tonal answers, or the answer (or portion of) occurring a step below the customary dominant level occur when the 5th degree of the scale, or the dominant chord, occur early (within the beginning) in the subject. This is to avoid the abrupt harmonic change which occurs in the answer if it were a real transposition to the dominant (V of V chord in the case of the dominant being implied in the subject). Even when the 5th degree appears early and does not imply a dominant chord, it is customary to use a tonal answer. This is a practice which was carried over from the days of modal music where the imitation was to be kept within the range of the mode. The most famous example of a tonal answer is the BWV 565 Fugue in D minor of Bach (toccata and fugue in dm), where the 5th degree occurs so often throughout the whole subject, Bach gives it a fully tonal answer in other words completely in the subdominant key.