But the basic trio sonata was usually written for two solo melodic instruments and bass continuo, three parts and instruments. In many cases, there would also be in the ensemble a bass instrument such as cello or bassoon that played the single notes of the continuo part, along with the continuo played on an instrument capable of harmony such as a keyboard or lute, and the two melodic solo instruments. Musical styles changed and the practice of basso continuo was considered old fashioned so composers wrote out their music for specific numbers of instruments, with the string quartet becoming a standard ensemble.
Early instances of works for 4 string instruments with no continuo were the sonate e quattro of the
Italian composer Allesandro Scarlatti, written in the early 18th century. From these early examples as well as others written by various composers, Joseph Haydn added his imagination and skill to form a standard that was popular with amateur musicians. Mozart was inspired by Haydn and added his genius to the form as well. It was the standards in artistry set by Haydn and Mozart in string quartet writing that inspired Beethoven to write his own string quartets.
The String Quartet No. 5 In A Major of the set was intentionally modeled on a quartet by Mozart (K.464) that was in the same key and followed the same general outline. It is in 4 movements:
I. Allegro - The first theme is a light tune that is contrasted by a second group of themes that delves into the minor and is punctuated by motives that follow one another in counterpoint. There is a summing up before the exposition is repeated. The development section begins with a variant of one of the motives of the second theme group until the first theme is then dealt with. Beethoven seamlessly leads into the recapitulation. A short coda ends the well constructed and tuneful first movement.
II. Menuetto - A graceful minuet has a bit of individuality thrown in by the way of dark minor rumblings towards the end of the first statement. The trio throws the ear a curve by accenting the third beat in the measure.
III. Andante cantabile - Mozart's example of a set of variations is also followed by Beethoven in the third movement, but the music is all Beethoven. The theme is a simple one in D major that rises and falls, with not much to recommend it as the basis for a set of variations:
The second variation has the first violin play an elaborate version of the theme while the other instruments offer a simple accompaniment.
The third variation has the second violin chatter a simple accompaniment while the other instruments comment on the theme.
The fourth variation is a contemplative variation on the theme that is high lighted by passages in the minor.
The fifth variation picks up the pace as the cello oompahs the bass line while the viola and second violin play a rhythmic variant of the theme. The first violin plays trills above them, and joins them a few times.
A coda continues to comment upon the theme in a slower pace until the music rises to a forte. After a brief pause, the first violin slowly plays the theme, the other have their final say, and the music ends pianissimo.
IV. Allegro - The final movement begins with a theme that finds the instruments chasing each other until the second theme group begins in a more hushed tone. The development takes up the first theme and takes it afield in key and mood. Parts of the secondary theme group are interjected until the recapitulation begins. A coda deals with themes once more before the music ends very softly and rather suddenly.