Flue pipes may be metallic or wooden. Metal pipes are usually circular in cross section; wooden pipes are usually square or rectangular, though triangular and round wooden pipes do exist. A flue pipe has two major parts, a foot and a resonator. The "foot" is the bottom portion of the pipe, usually conical. At its base is the "toe hole", through which wind enters it. The length of the foot does not affect the pipe's pitch, so organ builders vary the foot lengths of their flue pipes depending on other factors, including the desired shape of the pipes in the façade, the height of the rackboard in which the pipes are seated, and the weight of the completed pipe.