These are, as in the rondos, a Principal theme (called principal because it appears first, and thus becomes in a sense the index of the whole movement), and a Subordinate theme (so called in contradistinction to the other),--contrasting in character, as usual, but actually of equal importance, and of nearly or quite equal length.

To these, there is commonly added a codetta (or "concluding theme" as it is {122} sometimes called, though it seldom attains to the dignity of a _theme_),--sometimes two, or even more, codettas, which answer the general purpose of a coda, rounding off and balancing this Division of the design.

This union of the two or three thematic components that are to represent the contents of the design, is the _Exposition_, or first Division, of the Sonata-allegro forms.
It indicates a point of contact between the latter and the rondo,--in the _Third_ form of which we also find an Exposition.

Careful comparison of the two types of exposition reveals the significant difference between the two classes, however; in the Third Rondo, the exposition was an _alternation_ of themes, with decided preference for the principal one; in the Sonata-allegro it is a _union_ of themes, without preference, resulting in a broader thematic basis.
THE SONATINE FORM .-- In the Sonatine-form, or the smaller variety of the sonata-allegro designs, this Exposition (or first Division) is followed _at once_,--or after a few measures of interlude, or re-transitional material,--by a Recapitulation of the Division, as was seen in the Third Rondo-form, and under the same conditions of transposition as there.

The diagram of the form is therefore as follows:-- Exposition.