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 in satisfactory music form.
Of the Rondo-forms there are three grades, distinguished respectively _by the number of digressions_ from the Principal theme:-- The First Rondo-form, with one digression (or Subordinate theme), and one return to the Principal theme; The Second Rondo-form, with two digressions, and two returns; The Third Rondo-form, with three digressions and three returns.

The persistent recurrence of the Principal theme, something like a refrain, and the consequent regular alternation of the chief sentence with its contrasting subordinate sentences, are the distinctive structural features of the Rondo.
{108} THE FIRST RONDO-FORM .-- This consists, then, of a Principal theme (generally Two-Part or Three-Part Song-form); a Subordinate theme in a different key (probably a smaller form); a recurrence of the Principal theme (usually more or less modified or elaborated); and a coda.
Thus:-- _Principal Theme.