A tubular instrument which produces sound by the action of air against a reed or against the pipe mouth, in combination with the resonance of the air within the tube itself. Pipes are generally referred to as having a pitch or tonal length expressed in feet, such as 2′, 4′, 8′, etc. Pipes are usually either open or closed (stopped) at the top. Generally, the tonal length of a stopped pipe is twice that of a comparable open pipe (the same pitch sounded by a 2′-long stopped pipe would take a 4′ long open pipe to produce). Pipes used in automatic musical instruments are of two main types: flue pipes which produce their sound by the action of air against the edge of the pipe mouth in combination with a column of air; and reed pipes which produce sound by the action of a vibrating reed. There are two types of reeds: free and beating. The reed sound is amplified by the upper part of the pipe which, on a reed pipe, is called a resonator or horn. A third type of pipe, the diaphone, produces sound by intermittent bursts of air which are rapidly introduced into the base of the pipe under high pressure. Used in theatre organs. (Also refer to flue pipe, and reed, definition 2.) Pipes are mounted on a pipe chest or wind chest and are blown by air under pressure. Pipe pressure is measured by the number of inches that a given pressure will force a column of water up an open glass tube. (Pressure for calliopes is sometimes measured in pounds per square inch-the amount of pressure that the air will exert against a one-inch square area.) Pipes are tuned by adjusting the vibrating length of the reed by moving a tuning wire, or by changing the tonal length of the pipe by moving a tuning slide or stopper. Pipes are voiced to a specific wind pressure and cannot be interchanged with an instrument having significantly different pressure. Pipes are arranged in ranks and are controlled by registers or stops. Definitions of specific ranks are given elsewhere in this section. Refer to flute, piccolo, etc. for additional information. Often fanciful names ("fanfare trumpet," for example) were used to describe basic ranks. Certain ranks in large orchestrions are called foundation or fundamental pipes. These provide a rich bass sound which makes the treble pipes and solo pipes sound richer and fuller. Foundation pipes are never played alone but are always used in combination with other ranks. Solo ranks are those pipes with distinctive voicing which are used to play solo parts (or to carry the basic musical theme) while other ranks play accompaniment.