Theme. The form and contents of this factor, the Theme, are so
variable that a precise definition can scarcely be given. It is a
musical sentence of very distinct character, as concerns its melodic,
harmonic and, particularly, its rhythmic consistency; and of sufficient
length to establish this individuality,--seldom, if ever, less than an
entire period or double-period; often a Two-Part, not infrequently a
complete Three-Part Song-form, though never more than the latter.

In the Rondo-forms, two or three such Themes are associated in such
_alternating succession that, after each new Theme, the first or
Principal Theme recurs_. The term "Rondo" may be referred to this
trait, the periodic return of the Principal theme, which, in thus
"coming round" again, after each digression into another theme, imparts
a characteristic circular movement (so to speak), to the design. In
the rondos, then, all the movements of musical development revolve
about one significant sentence or theme, the style of which therefore
determines the prevailing character of the whole composition. This,
which is naturally called the Principal theme, is placed at the
beginning of the rondo. Its end being reached, it is temporarily
abandoned for a second sentence, called the Subordinate theme, of more
or less emphatically contrasting style and of nearly or quite equal
length (generally shorter, however), and always in a different key.
After this there occurs the momentous _return to the beginning_,--the
most insistent and vital fundamental condition of good, clear, musical
form, of whatsoever dimension or purport,--and the _Principal_ theme
reasserts itself, recurring with a certain degree of variation and
elaboration (occasionally abbreviation), thus vindicating its title as
Principal theme, and stamping its fellow-theme as a mere digression.
After this,--if a still broader design is desired,--another digression
may be made into a new Subordinate theme, in still another key,
followed by the persistent return to the Principal theme. And so on.
Upon the Subordinate theme, or themes, devolves the burden of variety
and contrast, while the Principal theme fulfils the requirements of
corroboration and concentration. A coda, sometimes of considerable
length, is usually added; it appears to be necessary, as a means of
supplying an instinctive demand for balance, increased interest, and
certain other scarcely definable conditions of very real importance