The entire orchestra proclaims the key of C major.
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Following such a dramatic and mysterious transitional leading, one which does not pause from the third movement to the last one, the entire orchestra thunders the first theme triumphantly.
The key has changed from a soft minor to a blazing major. Here, Beethoven includes not less than three trombones (which were hitherto silent), as well as a piccolo and double bassoon. Preceding Beethoven's time, these instruments were used in opera and ballet scores, but not in symphonies. In this movement Beethoven paves the way towards the larger orchestra, which will characterize the Romantic era.
The opening of the theme is fanfare-like, based on an ascending C major chord ("Triad"), and continues with a descending scale (from tonic, up to dominant, and back down to tonic). This gives full force to the victory of C major over C minor.
Nearly all the instruments of this enlarged orchestra play the opening of this brilliant theme in unison. This provides the feeling of a great victory, of optimistic exaltation over long struggles and a somewhat pessimistic uncertainty.
