Horn (musical instrument)
By the 1830s the modern horn with three valves had been invented.  These valves change the length of the tube, so that the horn had now become chromatic i.e. it could play all the notes including sharps and flats.  Composers like Richard Wagner and Richard Strauss wrote complicated music for the horn which would never have been possible on the natural, valveless instruments.  However, some composers preferred the sound of the natural horn.  Brahms wrote for the natural horn when he wrote his Horn Trio for piano, violin and horn.  Benjamin Britten, in his Serenade for tenor, horn and strings, asks for the horn to played naturally (without valves) for the fanfare at the beginning to make it sound like a hunting horn.