Orchestral horns are traditionally grouped into "high" horn and "low" horn pairs. Players specialize to negotiate the unusually wide range required of the instrument. Formerly, in certain situations, composers called for two pairs of horns in two different keys. For example, a composer might call for two horns in C and two in E♭ for a piece in C minor, in order to gain harmonics of the relative major unavailable on the C horns. Eventually, two pairs of horns became the standard, and from this tradition of two independent pair, each with its own "high" and "low" horn, came the modern convention of writing both the first and third part above the second and fourth.
In the mid-18th century, horn players began to insert the right hand into the bell to change the effective length of the instrument, adjusting the tuning up to the distance between two adjacent harmonics depending on how much of the opening was covered. This technique, known as hand-stopping, is generally credited to Anton Joseph Hampel around 1750, and was refined and carried to much of Europe by the influential Giovanni Punto. This offered more possibilities for playing notes not on the harmonic series. By the early classical period, the horn had become an instrument capable of much melodic playing. A notable example of this are the four Mozart Horn Concerti and Concert Rondo (K. 412, 417, 477, 495, 371), wherein melodic chromatic tones are used, owing to the growing prevalence of hand-stopping and other newly emerging techniques