Each stop controls a row of pipes, called a “rank”. Each rank makes a different sound (one row for the “diapason” sound, another row for the “flute”, another for the “trumpet” and so on). The stops control the air flow through the ranks. Some stops may control more than one rank. For instance, a Mixture stop of three ranks will have 182 pipes (3 ranks of 61 pipes each) and in some organs the Celeste is a 2 rank stop. The celeste pipes are tuned slightly sharper than the rest of the organ so that, when played together with another quiet stop such as the Salicional, there will be a pleasant throbbing beat because two pipes are slightly out of tune with one another. Organ Pipes are normally made of metal or wood. High quality metal organ pipes usually contain 75 percent tin or more, and the rest is lead. The Pipes are placed on windchests inside an "organ case" in a special room called an Organ chamber. A windchest is a box-like device which contains pallets that are opened and closed to admit air to a pipe so that it sounds. The pallets are operated by pull wires and rollers in the case of a tracker instrument but may also be operated by pneumatics or direct electric action using magnets.