Natural horn is the valve-less predecessor of the modern horn. Valve-less brass instruments are restricted to the notes of the overtone series by the laws of acoustics. This meant that the only way to play melodic passages was to play in the extreme upper register. Eventually, horn players devised methods to overcome this obstacle and play melodically in what we have come to think of as their most characteristic range. They achieved this by inserting their right hands into the bell and shading the open notes. This technique relied upon the players ability to match the tone quality of the closed and open notes as much as possible, and is the reason we still use our right hands in the bell of the modern horn to control both tone quality and pitch. This technique worked well for tonal music, with the horn pitched in the key of the piece (i.e. Concerto in D is written for horn in D), but it had severe limitations in more chromatic music. It was really only practical to make the sound of the closed and open notes match up enough in a very limited (and soft) dynamic range. To change keys a system of removable crooks was developed. A crook is a lead-pipe with enough tubing to make the horn play in a certain key. Later developments moved the crooks to the tuning slide so that the lead-pipe could remain fixed in place (inventionshorn). Below is a picture of my natural horn with it's crooks.
For more information on the history of the horn, and its technique, I would recommend the following sources:
Fitzpatrick, Horace. The Horn and Horn-Playing: and the Austro-Bohemian Tradition from 1680-1830. London: Oxford University Press, 1970. (Unfortunately, out of print, but check your library)
Janetzky, Kurt, and Bernard Brüchle. A Pictorial History of the Horn. Tutzing: Hans Schneider, 1976.
Morley-Pegge, Robert. The French Horn: Some Notes on the Evolution of the Instrument and of its Technique. London: Ernest Benn Limited, 1960.
Tuckwell, Barry. Horn. Yehudi Menuhin Music Guides. New York: Schirmer Books, 1983.
John Q. Ericson's articles online
Horn FAQ (Ron Boerger's Site)
Hans Pitzka's Site
These are only a few of the many sources available. If you are interested in natural horn technique the excellent modern editions/reprints of the tutors by Dauprat, Duvernoy, and Kling would be the most obvious place to start. For articles The Horn Call, and The Brass Bulletin both provide a rich source of material. The articles on horn in Grove, and MGG (in German) are also excellent resources, and would make an excellent starting point for anyone beginning their study of the history of the instrument.
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