W.A. Mozart
Symphony No. 40 in G Minor, K. 550

The conductor for the premiere was Antonio Salieri

The work originally did not contain clarinets, but the version that we hear nowadays has 2 clarinets.

No explanation exists as to why, but there are some suggestions. It is also unsure whether Mozart heard this work performed live, or if it was performed during his lifetime at all.
The addition of the clarinets to the score involves an addendum (a supplement) to the score, not a rewrite or reworking of the score. The clarinet part is largely a recast or redistribution of the oboe parts.
Mozart had a friend, Anton Stadler, who played the clarinet, and for whom the famous Clarinet Concerto, as well as a Quintet, and a Trio with viola and piano were written. Stadler was almost certainly the inspiration for the revisions of the symphony. Mozart would not have put himself through the trouble of the revisions except with a specific performance in view.

The G Minor Symphony score suggests "urgency" above all. Robert Schumann speaks of this work's Greek grace.

Mozart's voicing for instruments is a wonder in its subtlety. He never duplicates notes in the melody with notes in the accompaniment.
The repeat of the first melody is more a continuation and a development, since it is heard with oboes and bassoons supporting.
Other points of interest are the new atmosphere that is generated when the cellos and basses first play sustained rather than detached notes.

Theme 1 is urgent.
Theme 2 is all pathos. A subtle dialog between strings and woodwind, momentarily disturbed by fever, then charges to its conclusion.

The final chord of the exposition leads into an ambiguous chord in the development. The flute, oboe, and bassoon take that chord to a new modulation in a "very remote place indeed", F Sharp Minor.
Moments later, urgent intensifications are on the agenda.
The development works on the opening theme, and focuses more and more closely on its first three notes. The harmony becomes more chromatic and more anguished until a long descent - flute in dialog with clarinets - sends us into the recapitulation.
The second theme is dark - in G Minor.

The opening of the Andante has not as much a theme as a texture. Violas begin with an idea in repeated notes, but second and first violins join them in imitation at successively higher pitches.
The response is "sweet and graceful melody".
Mozart continues - more quietly - to explore the first movement's world of aching chromatic harmony. For the little descending two-note figures that are such prominent features here, the eighteenth century calls "sighs".
Polyphony is used in the first movement, and comes back in the Minuet. In the minuet's pastoral trio, the only time in this symphony that we settle in G major (the trio is also the only part of the symphony that was not rescored).

The finale brings the "most explosive music Mozart ever wrote". Eight measures of rude octaves and frozen silences that launch the development. Music is more consistently regular in rhythm than any we have heard in the first two movements. The normalty of most of the finale and the sense of direct momentum it generates that most establish the difference between the finale and the first Allegro. The first movement raises questions, instabilities, opens abysses. But for all the anguish Mozart still feels and expresses, the finale music at the last be a force that stabilizes, sets solid ground under our feet, seeks to close woulds, and brings the voyager safely - if bruised - into port.