What gave definite shape to this fundamentally simple form is the Italian love of melody. So far as it may be traced to the influence of one man, it may be traced to Giovanni Pergolesi, whose trio-sonatas first gave to the world as a prototype of the classical triplex form what is now known as the ‘singing allegro.’ Pergolesi was born in 1704 and lived to be only thirty-three years old; but in that brief life, gaily and recklessly squandered as it seems to have been, he exerted an influence upon the growth of music which apparently started it upon a new stage. He was all but worshipped by his countrymen. His opera, La serva padrona (1733), won instant success, not only in Italy, but well over all Europe; and had an influence comparable to that of but few other single works in the history of music. On the ground of instrumental music his trio-sonatas have, as it seems now, accomplished scarcely less.
We must here restrict ourselves to the harpsichord music of the time in Italy, in which the ‘singing allegro’ found place almost at once. Let us first consider what lay at the bottom of the new form.
We may plunge at once to the very foundation, the harmonic groundwork. As we have seen, perhaps the most important accomplishment in music of the seventeenth century was the discovery and establishment of key relationships in that harmonic conception of music which has endured almost to the present day. Instrumental forms developed upon this re-organization of musical material. Subsequently, however polyphonic the texture of a piece of music—a fugue of Bach’s, for instance—might be, its shape was moulded upon a frame of harmony. The piece was in a certain key, clearly affirmed at the beginning and at the end, points in the structure which in a piece of music as in a paragraph are naturally the most emphatic. Within these limits there was the life and variety of a harmonic development, which, departing from the tonic key, must return thence. Long before the year 1700 the regulation of such harmonic procedures had definitely fixed the symmetrical plan of two forms: the so-called aria form and the binary form. Neither was in itself capable of much development; and it was in a sort of fusion of both that the harmonic plan of the triplex form was created.
The aria form was in three sections which we have elsewhere represented by the letters A, B, A. A, the opening section, was all in the tonic key, and was practically complete in itself. B, the second section, was in a contrasting key or was harmonically unstable. A, the third section, was but an exact repetition of the first, to give balance and unity to the whole. The limitations of the form were essentially harmonic. The first section offered little or no chance for modulation. Its tonality must be unmistakably and impressively tonic. Therefore it did not develop into the second section by means of harmonic unrest. The second was simply a block of contrasting harmonies, like a block of porphyry set beside a block of marble. Frequently, however, the second section was incomplete without the third. In such cases a hyphen between the B and the second A in our lettered scheme would represent the relations between the three sections more nearly, thus: A, B-A.
The binary form, in which most of the dance movements of the suite were composed, was usually shorter than the aria form; but though apparently simpler, it was, from the point of view of harmony, more highly organized. It consisted, as we have seen, of two sections, each of which was repeated in turn. The first modulated from the tonic key to the dominant or relative major; the second from that key back to the tonic again. It will be observed that the first section really grew into the second by harmonic impulse; for the first section, ending as it did in a key that was not the key of the piece, was incomplete. The two sections together not only established a perfect balance of form and harmony, but had an organic harmonic life which was lacking in the aria.
However, the tendency of most forms was towards the triple division typified by the aria, with a clearly defined first section, a second section of contrasting and uncertain character, and a third section which, being a restatement of the first, reestablished the tonic key and gave to the piece as a whole a positive order. In the binary forms of Froberger and Chambonnières there is the harmonic embryo of a distinct middle section; namely, the few modulations through which the music passes on its way from dominant back to tonic in the second section. It can be easily understood that composers would make the most of this chance for modulation as they became more and more aware of the beauty of harmony; likewise, that the bolder their harmonic ventures in these measures, the greater was their need to emphasize the final re-establishment of the tonic key. Ultimately a distinct triple division was inevitable, with an opening section modulating from tonic to dominant, a second section of contrasting keys and few modulations; finally a restatement of the first section, as in the aria, but necessarily somewhat changed so that the whole section might be in the tonic key. Such is the harmonic foundation of the triplex form.
Such a form makes its appearance in music very shortly after the beginning of the eighteenth century. It seems akin now to the aria, now to the binary form. One may suspect the latter relationship if the first section is repeated, and the second and third sections (as one) likewise. These repetitions are obviously inherited from the binary form. On the other hand, if these sections are not thus repeated, the piece resembles more nearly the aria.
Take, for example, an adagio from the second sonata in a set of twelve published by Padre Martini in 1742, written probably many years earlier. These sonatas were republished by Madame Farrenc in the third volume of her Trésor des pianistes. The adagio in question is clearly in three sections very like an aria, with the difference that the first section ends in the dominant (in the eighth measure), and the last is consequently changed from the first so that it may end in the tonic. There are no repeats.
Far more remarkable is a sonata in C major by D. Scarlatti. It is the eleventh in the Breitkopf and Härtel collection of twenty to which reference has already been made. Here we find a first section modulating from tonic to dominant. This is repeated. Then follows the second section, full of free modulations, and this section comes to a very obvious half-close. The last section very nearly repeats the first, except for the necessary changes in harmony so that all may be in the tonic key. Scarlatti nowhere else wrote in this form so clearly. Did he merely chance upon it? The wide crossing of the hands marks an early stage in his composing, yet the form is clearly triplex and astonishingly orthodox.
The most striking aspect of this little piece is the obvious, clear divisions of the sections. The first section is marked off from the second by the double bar for the repeat. There is a pause before the third section, or restatement, begins. But clearest of all is the arrangement of musical material. By this we know positively that the triplex form has become firmly fixed, that the old binary form has expanded to a ternary form, submitting to the same influences that had made the perfect aria and the perfect fugue.