A flue pipe (also referred to as a labial pipe) is an organ pipe that produces sound through the vibration of air molecules, in the same manner as a recorder or a whistle. Air under pressure (referred to as wind) is driven down a flue and against a sharp lip called a Labium, causing the column of air in the pipe to resonate at a frequency determined by the pipe length. See wind instrument. Thus, there are no moving parts in a flue pipe. This is in contrast to reed pipes, which are driven by a beating reed, as in a clarinet. Flue pipes are common components of pipe organs.