All organ pipes are divided into two general classes, labial and lingual pipes. The main difference between the two classes is the manner by which the vibrations producing the sound are caused. In labial pipes the column of air entering the pipe under pressure is set in vibration by a fixed obstruction at the mount of the pipe. […] [T]he obstruction at the mouth […] causes the column of air to vibrate. In the lingual or reed pipe, the vibrations are caused by the air passing through a metal reed, which causes the tongue of the reed to vibrate, thereby setting up sympathetic vibrations in the column of air in the pipe.