An organ pipe is a sound-producing element of the pipe organ that resonates at a specific pitch when pressurized air (commonly referred to as wind) is driven through it. Each pipe is tuned to a specific note of the musical scale. A set of organ pipes of similar timbre comprising the complete scale is known as a rank; one or more ranks constitutes a stop. Construction Materials Organ pipes are generally made out of either metal or wood. Very rarely, glass, porcelain, plastic, paper, Papier-mâché, or even stone pipes may be seen. A historical organ in the Philippines has pipes made exclusively of bamboo. Metal Metal pipes are usually made of lead; for increased rigidity it is alloyed with tin along with trace amounts of antimony and copper. The percentage of each metal in the alloy influences the characteristics of the resulting pipe. A high proportion of tin results in a slightly brighter colour (optical colour, not timbre). In addition, high amounts of tin give a gleaming and long-lasting polish, which may be desired if the pipe is clearly visible. The cost of each metal is also a factor, as tin is more expensive than lead. Cost considerations may also lead to the use of the inferior rolled zinc especially for the lower tones that take a lot of material. In addition, pipes have been made of many metals, including copper, aluminium, gold electroplate, silver, brass, and iron. Metal pipes are generally made by first casting the desired lead alloy onto a long flat surface. Once the metal cools, it is cut into pieces, which are then rolled into shapes around molds called mandrels and soldered together. Thus, the cross-section of a metal pipe is usually circular. The low melting point, solderability and malleability of the organ metal makes the construction of pipes relatively easy. Wood The body of a wooden pipe can be made of either a coniferous wood (softwood) or hardwood, although the lower section of the pipe (comprising the metal foot (on some pipes), cap, block and mouth) will nearly always be made from hardwood to provide a precise edge for the pipe's mouth. Using screws and glue, the pipes are assembled from wooden pieces of various shapes and sizes. In contrast with the circular cross-section of a metal pipe, the cross-section of a wooden pipe is most commonly square or rectangular. Glass Glass pipes have been