A FrenchHorn is a musical instrument. 

Modern FrenchHorn''''''s are a perfect example of TheMostComplexWhichCanBeMadeToWork, usually consisting of two or three independent instruments sharing an entrance and exit, and selected with two or three thumb-operated levers, as well as having three or four levers operated by the first through third or fourth fingers, usually on the left hand. It is played facing backwards (the instrument, not the person) and with the right hand in the bell, usually. Yet, somehow, it produces one of the broadest spectrums of colors of all the instruments, save synthesizer and full drum section. In the hands of a wizard, it is the Voice of God. In the hands of most mere mortals, it's just plain dumb sounding. 

Mozart especially liked them and wrote some of his nicest music for them.
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 I once had a whim and I had to obey it
 To buy a FrenchHorn in a second hand shop
 I polished it up and I started to play it
 In spite of the neighbours who begged me to stop

 (interlude)

 To sound my horn, I had to develop my ''embouchure''
 I found my horn was a bit of a devil to play
 So artfully wound
 To give you a sound
 A beautiful sound so rich and round
 Oh, the hours I had to spend
 Before I mastered it in the end

 ...

From memory, by FlandersAndSwann, sung to Mozart's Horn Concerto No. 4 in E-flat major, K495.
(http://ourworld.compuserve.com/homepages/mshanemcl/mozart4.htm)
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If I recall correctly, a competent french horn player can play the entire musical scale without using any of the keys.

''A competent player of any brass instrument can do this on his or her instrument. That's not to say it sounds good, or that there's any point to doing it, but it can be done. It's a matter of manipulating the lips and mouth to play the tones which lie between those of the harmonic series above a given fundamental tone.''
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I think there is a point to doing it.  One time I heard Alan Civil play the final movement of the Mozart fourth horn concerto on a valveless "Waldhorn" -- the intonation was unusual, and perhaps a little odd for modern ears accustomed to the wide major thirds, etc. of equal temperament.  But it was different, and charming.  At all hazards, we do well to remember that Ignatz Leutgeb, the musical cheesemonger for whom Mozart wrote his works for horn, played just such a valveless instrument.  The so-called "Wagner tuba" wasn't even a gleam in anyone's eye in those days.  
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