'_ the first subject slightly modified and _b_ the
answering material, and _b'_ the answering material of the
counter-theme somewhat modified, the lyric period would present the
following schedule:

_a_ plus _b_ _a_ plus _b'_;

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.

This is the form of the great majority of polkas and waltzes, except
that the song forms standing for A and B respectively are very often of
three periods instead of two. This