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The trios of the seventeenth century—the Sonate a tre—were written for three concertizing instruments and a figured bass, really four parts in all. During the eighteenth century the word trio took on quite a different significance and was applied to compositions written for the harpsichord with one other solo instrument, violin, oboe, or flute, like the violin sonatas of Bach. Vaguely at the time of the young Haydn, clearly when Mozart entered the world of music, the word took on the meaning that it still holds today: a composition written for three instruments, pianoforte, violin and cello. If another combination of instruments is meant, then those instruments are usually specifically designated in the title of the work.

The Haydn Trios are of little importance. There are thirty-five in all, and it has been said that the majority were written for a patron who played the cello a very little. Hence one finds the cello part in this combination to be merely a duplication of the bass part of the pianoforte, having little independent movement of its own; and the works are rather sonatas with violin than trios.

Mozart, on the other hand, treated the combination with a fine sense of the effects that could be made with it. He gave to each of the three instruments a free line of its own, and made fine use of the possibilities of tonal contrast and color. There are eight trios in all. They are not representative of Mozart’s best, though there is not one in which Mozart’s inimitable grace is lacking; but in spite of their slenderness they may be considered the first pianoforte trios in the modern sense, and to have set the model for subsequent works in that form.

These are not very numerous, if one excludes from them a great number of fantasias or popular operas such as were written by Woelfl, Nicholas Lomi and other composers of the virtuoso type. Nor does the form show much development except that which accompanies an improvement in pianofortes and a progress in technical skill on all these instruments. Only a few trios stand out conspicuously as having high musical worth, or as having been a worthy expression of genius.

There are eight trios by Beethoven. Of these three were published as opus 1, and hardly show an advance over the trios of Mozart, if indeed they do not fall considerably short of them in point of finish and style. Two were not published in his lifetime, and one of these is only a fragment, a single movement in B-flat major, composed in June, 1812, for Maximilian Brentano. There are, then, but three that are representative of the mature Beethoven, two published as opus 70, and one, in B-flat major, opus 97, dedicated to his favorite pupil, the Archduke Rudolph. The writing for the three instruments is especially clear in the first allegro of opus 70, No. 1, a lively, vivacious movement in D major. The slow movement of this trio is rather remarkably scored for the pianoforte, which is almost constantly engaged in tremolos, strange broken trills, and runs. The last movement is full of Beethoven’s humor, very distinctly in the swing of a folk-song. Throughout there is much brilliant work for the piano, and a ceaseless witty interchange between the other two parts. There is an extraordinary pedal point before the return to the first section, which is just touched upon at the end. The second of this pair of trios is not less brilliantly arranged for the three instruments. The variations in the second movement are finer than the variations in the earlier works. There is folk-song again in the third movement, a smooth allegretto in A-flat major. Both trios are extraordinarily clear and happy in mood.

The trio opus 97 is one of the biggest of Beethoven’s works. The contents are more symphonic than those of his other trios, and recall something of the spirit of the quartets of opus 59. There is, indeed, a marked similarity between the opening theme of this trio and that of the quartet opus 59, No. 1, especially in the broad line of the melody. Yet though on the whole the effect of this great trio may be orchestral, there are not lacking measures of finest style, like those which follow the second theme in the first movement, with the touch or two of delicate imitation, then the soft melody of the cello with the dainty scale on the pianoforte, and then the cello and violin in octaves, with the scales on the pianoforte becoming more and more active and noisy. Immediately after, it is all cleverly changed about; the strings have those lively scales and the pianoforte the melody. The scoring of the whole Scherzo, too, is especially in trio style, and may well be taken as a model. The andante and variations, and even more the last movement, are, however, hardly in the style of chamber music, and the vigorous passion of the ideas in them does considerable violence to the essentially delicate combination.

The combination is without doubt one of the most difficult to treat with success, partly because the pianoforte may be very easily led to overpower its fellow instruments, partly because notes in the lower ranges of the cello have so little carrying quality that except in very soft passages they cannot be heard in the combination. It must be said that the general development in pianoforte technique did much to overthrow the balance and adjustment so charming in the trios of Mozart and in those of opus 70 by Beethoven. Between Beethoven’s last trio, opus 97, and the trios of Brahms there is hardly a single one that does not suffer from maladjustment.

The two trios of Schubert, opus 99, in B-flat, and opus 100 in E-flat, are full of inspiration, and Schubert’s fancy is so delicate that on the whole he may be said to have succeeded with the combination. Certainly the little canon which forms the Scherzo in the second trio is a masterpiece of style. Also the announcement of the chief theme in the first trio and the way in which it is developed cannot be found fault with; nor is the charming D-flat section in the finale less perfect. But in the scherzo there are rather weak accompaniments scored for the strings in the orchestral manner of double stops, and there are similar passages at the beginning of the transition to the second theme in the first movement of the second trio. These are here acceptable because of the sheer beauty of the material which is thus presented; but one cannot deny that this would find even lovelier expression with a group of three strings. In the Andante con moto the impropriety of style is more evident; but one will forgive anything in this inspired movement, which later is to stand like a shadow behind the Marcia in Schumann’s great pianoforte quintet.

Mendelssohn wrote two trios, one in D minor, one in C minor, which, after having for years been favorites with players and public alike, are now sinking out of sight. In these the treatment of the pianoforte is brilliant; and though it may not be said to overbalance the strings, it certainly outshines them. Mention should be made of Marschner’s trio in G minor, opus 110, because it so clearly influenced Schumann in his own quartet in A minor. Five trios of Spohr’s were once well known, but they represent no change or development either in style or form; and even that in E minor, opus 119, which has been prized almost to the present day because of its melodiousness, is fast being abandoned.