A stop is an individual voice in the organ, composed of one or more ranks of pipes. Its name includes a number, like 16',4', etc., which designates its pitch according to the length of its largest pipe (corresponding to low C on the keyboard). If it includes more than one rank of pipes it is a Mixture, with a Roman numeral in front of its name to indicate the number of ranks, as III or IV. The word stop is also applied to the control in the console which turns on the actual stop. The name comes from the original controls, which were introduced to shut off, or 'stop' some of the ranks so they did not all play at once, as they did in the earliest organs. See the Encyclopedia of Organ Stops for a information on specific stops.