It is observed that to use single F horn for high notes is difficult even with the valves. In High notes where the partial grew closer and closer, making accuracy is a great challenge. A quick solution is simply to use a horn of a higher pitch like Conn 8D French horn. The use of F relative to B horn was a focus of debate between horn players of the late nineteenth century until the German manufacturer Horn Ed Kruspe produced a prototype of the horn "double" 1897.

The double horn combines two instruments also in a unique setting: the original horn in F and a second, higher horn keyed in B like in Conn 8D French horn. Using a fourth valve (usually operated by the thumb), the horn player can quickly move from the 'deep warm tones of the horn from F to the higher, brighter tones of the Horn B. The two sets of tones are commonly called "camps" of the horn. The function of fourth valve is to change the length and therefore the harmonic series and height of the instrument; it also causes the three main valves to use proportionate lengths of slides

In the United States, the two main styles that are currently popular are for double horns are named Kruspe and Knopf, after the first instrument makers who developed and standardized them. The location of B valve is near the thumb valve which is situated above the first valve and wrapping Knopf. They have also changed the valve behind the third valve, near the little finger (although the trigger valve is still played with the thumb). Indeed, air flows in a completely different direction on the other model. Conn 8D French horn tends to be larger in the throat of the bell type Knopf. Typically, models are constructed from German silver or nickel, while type Knopf horns tend to be yellow brass. So, a horn section conductor is usually having one or the other because of differences in color tone, response, and projection of the two different styles.