The French horn is a brass instrument consisting of a coiled tube with a flared bell. Originally, the so called 'natural' horn was limited to producing the notes of the harmonic series of its base pitch (based on the length of the tube). Slight changes to intonation could be made using the lips or placing the hand inside the bell. During the early 19th century, a valve system was developed to allow horn players to change the length of the instrument, giving birth to the modern French horn. Nowadays horns are usually double, with two sets of tubing pitched to F and B-flat. These are switched using a fourth valve, and, combined with the standard three-valve system, allow the horn to reach virtually every note in its range.