Binary Form I
Binary Form consists of two equal clauses, set in antithesis and containing between them a modulation out of the tonic and back again. Of this there are three possible variants:
the first clause may be wholly in the tonic, and the modulation may be contained in the second;
the first clause may modulate, and the second may lie wholly in the tonic;
the modulation may be divided between the two clauses; the first leading out of the key, the second starting out of the key and leading back.
Of these three variants the last is structurally the most advanced. For in it neither clause can stand alone; each ends in a different key from that in which it began, and each, therefore, requires the co-operation of the other before the sense of unity can be produced by the melody as a whole.
But before this form can be adapted to the use of longer movements a second element must be added. It is not enough that the key system should be coherent; there must also be an intelligible recurrence of phrase. In a short melody, such as the three tunes quoted above, this device is unnecessary, since there is not enough material to cause any confusion; but in works of larger scale and compass the ear will obviously grow bewildered if it be made to follow a series of continuous changes. So, as instrumental music advanced, composers began to cast about for a method of unifying their phraseology, partly to counterbalance the increasing diversity of key system, partly to stimulate their audience by repeating passages which it could remember and recognize. A somewhat artless and ingenuous instance may be found in the following Gavotte from Corelli’s Sonata da camera a tre Op. 2 n. 1.
Here the scheme of modulation is as complete as the length of the tune will permit; but the phraseology is so uniform as to be almost monotonous. At the same time there are in it two forces that make for organization:
the division of each clause into two sub-clauses, distinct and definite in key system;
the final assertion in the tonic of a phrase which, at the half-way point, has been presented in the dominant.
If, then, the four sub-clauses can be so arranged and diversified as to form an intelligible pattern, the simple Binary form will be brought another stage toward completion.
In the Corelli’s tune quoted above the effort at unification has overshot its mark. The four sub-clauses are so closely similar in curve and rhythm, that the impression left by the melody as a whole is less one of design than that of reiteration. But suppose the first two sub-clauses contrasted in phrase as well as in key; the one presenting a short melodic sentence in the tonic, the other an entirely different sentence in the dominant. Then the design will be completed if the last part of the tune follows the phraseology of the first while it reverses the key system, giving in the dominant what the first part has given in the tonic, and viceversa. And thus we get a Binary scheme which may be expressed in the formula:
a1 – b2 || a2 – b1
where “a” and “b” represent the two sub-clauses, 1 and 2 the keys respectively of tonic and dominant (or relative major if the piece is in minor key).
A good example may be found in the opening movement of Benedetto Marcello Sonata Op. 1 n. 1 in F major. Here is quoted only the upper voice.
Note that the second part is extended by the insertion of a bar modulating back to the tonic, and that the second sub-clause, on its reappearance, has its opening bar slightly modified, though not enough to prevent its recognition. These licenses are clearly advisable, partly from the exigencies of the key, partly to ensure some variety and flexibility in the structure. As it stands, this movement is the central type and example of Binary form as commonly employed by the composers of the late XVII and early XVIII century. No doubt variants and experiments were tried, notably in Corelli’s famous Giga in A major (from n. 9 of the twelve Sonatas Op. 5), which repeats the first of the sub-clauses in place of the second. But as a rule the general outlines of the scheme were preserved intact. In longer movements there would be more incidental modulation, greater freedom of phraseology, occasional omissions or insertions of details, and the like; yet with all liberty of treatment there is very little variation of principle. In a word, then, this early system may be summed up as that of a poem in two cantos, approximately equal in length, the first beginning in the tonic and ending in the dominant or relative major; the second following the phraseology of the first and inverting its scheme of keys. Mechanically stated, the form is as follows:
Out of deference to the listeners, both parts were usually repeated.
Cfr. Hadow, Sonata form (Novello ed. 1896), Ch. 1 and 3.