Page:Fugue by Ebenezer Prout.djvu/156

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290. Hitherto our task has been comparatively easy. It is possible to give very definite rules as to the correct answering of a fugue subject, the treatment of the exposition and episodes, and the construction of stretto. But in dealing with those parts of a fugue which we are now approaching, we are met by far greater difficulties than any we have as yet encountered. These arise from the fact that as soon as in composing a fugue we get beyond the exposition (or counter-exposition, if there be one), we are left to a very great extent free to do what we please; and it is quite impossible here to give more than very general principles for the guidance of the student.

291. The first great fact which must be clearly grasped is, that every fugue, however much variety there may be in the details, is in its main outlines constructed in the same general form. This is the form which is commonly known by the name of Ternary or Three-part form. A movement in ternary form can always be divided into three principal sections. In a piece other than a fugue (for instance, in the slow movement of a sonata, in which this form is frequently used), the first section will be mostly in the key of the tonic, and will close either in that key, or in one of the most nearly related keys—probably the dominant if the movement be in a major key, and the relative major if it be in a minor key. The second part of such a movement generally consists of an episodical subject, but is invariably in a different key from the first part; while the third section usually repeats the subject of the first in the tonic key.

292. We can best show the ternary form by a diagram—

This skeleton form can be filled up in an almost infinite number of different ways, as regards variety of detail; but the broad outline given above can always be distinctly traced.