The words sinfonia and symphony were used for instrumental pieces in opera, concertos and sonatas all through the Baroque period in the 17th and early 18th centuries. Soon the term symphony and overture were used interchangeably in Italian opera. The overture to many Italian operas followed a three-movement form that had the tempo scheme of fast-slow-fast. The evolution of the form was more or less standardized with the later symphonies of Haydn and Mozart in the late 18th century by the addition of a fourth movement.
Scholars have determined that C.P.E. Bach wrote his first documented symphony in 1741. It followed the fast-slow-fast form and was for strings and continuo, which makes Bach one of the first recognizable composers of compositions that were meant to be heard as stand-alone works not tied to the theater. There are 20 symphonies extant that can be confidently attributed to Bach, all of them follow the three movement form.
The other feature of the symphony is the presence of sonata form, usually found in the first movement, but it can be used for any of the movements. It was derived from binary form, a form that consists of two related sections that are repeated. An early style of sonata form was used by Bach in his symphonies, with Haydn and Mozart standardizing a model of the form in their symphonies and other works. As C.P.E. Bach uses it, sonata form utilizes two themes, the first being in the tonic key, the next in the dominant key (or other related key), that are played in succession after which a short section develops these themes by variations in key, phrasing, etc. The themes appear again (recapitulation) after the development with the first theme played as before, and the second theme modulating to the tonic (usually).
Bach's Symphony In G Major Wq. 183/4 is in three movements:
I. Allegro assai - An angular, down-beat stressed theme is played by the violins while the second theme is played by the flutes with comments by oboes and violins. The first theme reappears and goes directly to the development section where the second theme is commented upon after which the first theme modulates to the minor. This leads to the recapitulation that segues to the slow movement
II. Poco andante - A short movement in the minor that serves as a contrast in mood to the first movement.
III. Presto - A lively dance movement.