A natural horn. Via Wikimedia Commons.
The horn that plays in the symphonies of Haydn, Mozart and the early Beethoven was a coiled tube and nothing more: no valves, no machinery. An instrument like that can produce only the natural harmonics of its length, a scale full of holes. And here one of the most ingenious inventions in the history of performance practice comes on stage: the right hand inside the bell.
The technique, codified in the mid-eighteenth century by the Bohemian Anton Joseph Hampel in Dresden, works like this: by closing the bell more or less with the palm, the pitch drops by a semitone or almost a whole tone, and the holes in the scale are plugged one by one. The price is timbre: the stopped notes sound veiled, a little nasal, different from the open ones. A horn player of the period was a tightrope walker who alternated ringing sounds and shaded sounds within the same phrase, and composers wrote knowing exactly which notes would come out in full light and which in half-shadow: that chiaroscuro was part of the argument, not a flaw. To change key, moreover, a piece of tubing was physically swapped, the crook, which is why in scores you read horn in E-flat, in D, in F: assembly instructions, close relatives of the clarinet’s transposition.
Valves arrive around 1814-18, in Germany, and open the way to the modern horn, chromatic and uniform. But the transition was anything but immediate, and the most delightful case is Brahms: he wrote to the end, throughout the second half of the nineteenth century, with the natural horn in mind, which he had learned as a boy, and the famous solo that opens his Trio, Op. 40, is conceived for that world of lights and shadows, even though today it is almost always played with valves. Orchestras that perform on period instruments restore that chiaroscuro, and it is one of the differences you hear at once, even as a layperson: the horn stops being a smooth surface and becomes a landscape again, with sun and clouds. The next time a horn in a classical symphony seems to change character from one note to the next, it isn’t your imagination: for a century, that character was decided by a fist.