 an organ stop (or just stop) is a component of a pipe organ that admits pressurized air (known as wind) to a set of organ pipes. its name comes from the fact that stops can be used selectively by the organist; some can be "on" (admitting the passage of air to certain pipes), while others can be "off" (stopping the passage of air to certain pipes). the term can also refer to the control that operates this mechanism, commonly called a stop tab, stop knob, or drawknob. the term is also sometimes used as a synonym for register, referring to rank(s) of pipes controlled by a single stop. registration is the art of combining stops. ==mechanics== thumbnail|300px|right|stop tabs on an electronic organ, located above the uppermost manual. compare this placement to that of the stop knobs on the pipe organ console pictured below. organ pipes are physically organized within the organ according to note and timbre, into sets. a set of pipes producing the same timbre for each note is called a rank, while each key on a pipe organ controls a note which may be sounded by different ranks of pipes, alone or in combination. the use of stops enables the organist to selectively turn off ("stop") certain ranks in order to produce different combinations of sounds, as opposed to hearing all sounds simultaneously. while nowadays one speaks of "drawing" a stop to select a particular rank, the earliest organs were constructed as blockwerk with all ranks 'on' by default. the mechanism used to operate the stops varies widely, but the principle is the same: the stop control at the console allows the organist to select which ranks of pipes will sound when a key is pressed. when the organist desires a rank to sound, he or she operates the corresponding control at the console, allowing wind to flow to the pipes. likewise, the organist can deny wind to the pipes by operating the same control in the opposite direction. common stop controls include stop knobs, which move in and out of the console, and stop tabs, which toggle back and forth in position. some organs, particularly smaller historical organs from england or spain,james dalton, "iberian organ music before 1700," in the cambridge companion to the organ, ed. nicholas thistlethwaite and geoffrey webber (cambridge: cambridge university press, 1998), 165. feature divided registers, in which there are two stop knobs for certain ranks. one stop knob will control the upper portion of the keyboard, and the other will control the lower portion 
Musical_instrument_parts_and_accessories Organ_stops Pipe_organ_components