and the thematic this:
_a_ plus _a'_ _b_ plus _b'_;
or, more generally:
_a_ plus _b_.
The a and b in this latter case each extend to four measures.
In case a form is to be developed to two periods, new material is often introduced at the beginning of the second period. Designating this new material by c and c', the schedule of the two-measure period would be as follows:
First period: _a_ plus _b_ _a_ plus _b'_.
Second period: _c_ plus _c'_ _a_ plus _b'_.
Thus represented in algebraic formulae, it is easy to see that repetition of the materials designated a, or a and b together, is the source of unity in the period, and the third element introduced, here designated as c, has its only use in serving as variety. The normal dimensions for the two-period form just scheduled would be sixteen measures; but if the motive were two measures, then the period form resulting would be sixteen measures, and the two-period form thirty-two measures. Many examples will be found in the instrumental works of Haydn, Mozart, and Beethoven, and also in Schumann.
This simple form above given serves also as a type of the organization of the larger forms. For example, one of the most numerously represented forms in music is the rondo, which derives its name from the reappearance of the principal subject at intervals, after the manner of a round. Supposing such a principal subject to be a one- or two-period song form like those described above, this entire form would be designated as A; after A, a small amount of passage work might be introduced, and then would enter a second form, B, which within itself, however, would be modeled quite like the two-period form described above. After this second form the first form would then be repeated, and after this a coda would be added. Designating the entire first form or principal subject of a rondo by A, and the second subject or second song form by B, the rondo then will have this schedule:
A plus B plus A plus Coda.