In the late 18th century, Viennese court trumpeter Anton Weidinger developed the world's first keyed trumpet. For the first time, the trumpet was no longer limited to the diatonic notes of the natural trumpet. Haydn wrote his trumpet concerto for Weidinger in 1796 making use of the new ability for the trumpet to play chromatically.
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The piece in written in three movements typical of the Classical concerto with the first movement in sonata form and the final movement in rondo. The piece is scored for an E-flat trumpet, and orchestra accompaniment to include strings, two flutes, two oboes, two bassoons, two horns, two (natural) trumpets, and timpani.
As is typical with the Classical concerto, the first movement features a lengthy introduction, in which Haydn inserts his first joke. Before the trumpet solo begins, it plays a short fanfare in the introduction that uses only the notes available to the natural trumpet. Audiences were expected to be disappointed at the traditional use of the chromatically capable instrument. Although the piece does use harmonic structure for the basis of the melodic line, Haydn mostly features the instrument's chromatic ability.
As is always the case with Haydn, the piece uses an extensive dynamic range that is often intended to grab the audience's attention. Harmonically, Haydn rarely strays from very Classical motion making use of I-IV-V-I and I-ii-V-I progressions. Also, the second movement is written in A-flat, the subdominant key relative to the first and third movements which is also typical of the Classical concerto.
The most obvious difference is the use of the keyed trumpet which is able to play chromatically. Secondly, because this was written late in Haydn's career, it is fully mature in its classical style with the bassoon separate from the continuo and the viola separate from the bass.
Again, as a trumpet player, I chose this piece because of the landmark that it provides in the instrument's history. Once an accompanying instrument used for militaristic fanfares, the trumpet became a major solo instrument with the development of keys/valves.
Bonds, Mark Evans. A History of Music in Western Culture. 4th ed. Upper Saddle River, NJ: Pearson Prentice-Hall, 2013.
Phillips, Edward. "The Keyed Trumpet and the Concerti of Haydn and Hummel: Products of the Enlightenment." International Trumpet Guild Journal no. 22 (2008): 22-28.