reed and the length of the protruding part. For the flue pipes it is determined by the shape of the air column inside the pipe and whether the column is open at the end. For those pipes the pitch is a function of its length, the wavelength of the sound produced by an open pipe being approximately twice its length. A pipe half the length of another will sound one octave higher. If the longest pipe, C, is in length, the pipe one octave higher will be long, and two octaves above (middle C) will be long. A closed (stopped) pipe produces a sound one octave lower than an open pipe. For example, a stopped pipe long will produce the same pitch as an open pipe 8 feet long: two octaves below middle C. The nomenclature of a rank of pipes is based on the size of an open pipe that would produce the same pitch, regardless of the type or size of the actual pipes in the rank. For example, a rank of open pipes labeled as 8′ (pronounced "eight-foot") would have a pipe for C two octaves below middle C that is approximately 8 feet long. An 8′ stop is said to sound at "unison pitch": the keys on the organ console produce the expected pitch (e.g. the key for middle C causes a middle C pipe to speak), like a piano. In a rank of stopped pipes, the lowest pipe is 4 feet in length but sounds at unison pitch—that is, at the same pitch as an 8′ open pipe—so it is known as an 8′ stop. Reed pipes are also labeled the same as that of an open pipe with the same pitch, regardless of the actual length of the pipe. Varieties Flue pipes The sound of a flue pipe is produced with no moving parts, solely from the vibration of air, in the same manner as a recorder or a whistle. Wind from the "flue", or windway is driven over an open window and against a sharp lip called a Labium. By Bernoulli's principle this produces a lower pressure region just below the window. When the vacuum under the window is large enough, the airstream is pulled under the Labium lip. Then the process works in reverse, with a low pressure region forming over the Labium which pulls the airstream to the other side again.