 The two sets of tones are commonly called "sides" of the horn.Using the fourth valve not only changes the basic length (and thus the harmonic series and pitch) of the instrument, it also causes the three main valves to use proportionate slide lengths.In the US, the two most common styles ("wraps") of double horns are named Kruspe and Geyer/Knopf, after the first instrument makers who developed and standardized them.