String trios for violin, viola and cello came about as a form roughly in the last half of the 18th century. They came from the earlier form of trio sonata for two or three solo instruments plus basso continuo. There were three parts to the earlier trio sonata, even if there were in actuality 3 soloists and continuo, as the continuo played the bass and harmonies and was always included. The continuo was most often a keyboard instrument, but the bass line itself was often doubled by a bass instrument such as the cello. The trio sonata designation came from there being three parts to the work, regardless if there were three, four or sometimes five performers. J.S. Bach and other Baroque composers wrote trio sonatas for organ where the right hand, left hand and pedals each have their own part.
The continuo was slowly done away with when music moved from counterpoint towards a melody with accompaniment. The first string trios were for two violins and cello, with a further development beginning with Haydn of violin, viola and cello.
Beethoven wrote a total of five string trios, all of them early in his career. The first two, Opus 3 and 8, are more in the style of the serenades of Mozart as they are in six and seven movements respectively. It is with the three trios of Opus 9 that Beethoven takes the form with more seriousness. The content of the works themselves and the fact they were written in 4 movements each show that Beethoven did not mean for them to be considered light entertainment as a serenade.
Beethoven wrote the trios of Opus 9 in 1797-1798 at a time when he was the toast of Vienna, mostly for his performances as a virtuoso pianist and improviser. He had been composing since he was still a child with a steady progression quality and artistry in his work. Most of his previous opus numbers involved the piano either as a solo instrument or with string soloists. There were a few other works for strings alone, but it was with the opus 9 trios that saw his ability to write for strings take on the qualities of a master. That they are seldom played anymore has nothing to do with the quality of the writing. Perhaps Beethoven himself considered these trios as a warm up to writing string quartets, a form that was viewed at the time (and still is) as the pinnacle of compositional artistry. After Beethoven wrote the six string quartets of opus 18, he never returned to the string trio.
While all three trios are worthy of listening, it is the third one in C minor that shows flashes of the Beethoven to come. The key of C minor is an important one in Beethoven's oeuvre, as some of his most dramatic and innovative music is written in that key.
I. Allegro con spirito - The first movement is in sonata form and Beethoven begins straight away with the three instruments playing a short motive in unison. The 1st theme is in C minor, and is repeated after the first hearing as the cello takes over the theme as the violin plays running 16th notes. The theme is cut short as the violin plays some syncopated chords that lead to the next theme. This 2nd theme is in E-flat major and is simply stated by the violin and cello, while the viola gives a feeling of tension with running staccato 16th notes. Roles are reversed in the repeating of the 2nd theme. There are other fragmentary themes played before the movement closes in the key of E-flat major. The movement is repeated. The development section begins with treatment of one of the lesser themes heard at the end of the exposition. Where the development section ends and the recapitulation begins is blurred by Beethoven's technique of bringing back the main themes of the movement in different instruments amid a bustle of activity. A coda ties up all the ends that Beethoven cares to, and the movement ends in C minor.
II. Andante con espressione- As impassioned as the first movement is, so is the second movement soft and sweet. Beethoven writes in 4 parts in C major in many places in this movement, which gives the music a fullness that belies that there are but three instruments playing. The music sings throughout, and ends quietly in C major.
III. Scherzo: Allegro molto e vivace- Beethoven returns to the home key for this tart and brisk scherzo. With sudden accents and bursts of sound, there is no trace of a minuet. A calm middle section in C major gives contrast. The scherzo returns and ends pianissimo.
IV. Finale: Presto - The tone of the scherzo and 1st movement continues with the finale written in sonata form. Scales, accents and extremes in dynamics give a hectic feel to the music. The trio ends quietly in the key of C major.