CHAPTER X .-- THE THREE-PART SONG-FORM.
DISTINCTION BETWEEN BIPARTITE AND TRIPARTITE FORMS .-- We learned, in the preceding chapter, that the Two-Part Song-form is a composition of rather brief extent, with so decisive a perfect cadence in its course as to divide it, in a marked manner, into two separate and fairly individual sections or "Parts." Between this and the next higher form,--that with _three_ such Parts,--there is a distinction far more essential and characteristic than that of mere extent; a distinction that does not rest simply upon the number of Parts which they respectively contain.

Each of the two classes of formal design, the Two-Part and the Three-Part, embodies a peculiar structural idea; and it is the evidence of these respective ideas,--the true content of the musical form,--which determines the species.

The "number" of sections is, in this connection, nothing more than the external index of the inherent idea.
The Two-Part forms embody the idea of _progressive growth_.

To the first Part, a second Part (of similar or related melodic contents) is added, in coherent and logical succession.

It should not be, and in good clear form it is not, a purely numerical enlargement, for the association of the second Part with a foregoing one answers the purposes of confirmation and of balance, and is supposed to be so effectuated as to institute and maintain unity of style, and some degree of progressive development.