One rank of pipes equals one stop. The drawknobs on the organ, which control the air flow to the ranks, are labeled with a name, such as Principal, and a number, such as 4'. The name describes the kind of sound and the number tells the pitch range. Pipes can be made of wood or metal. Both wood or metal pipes can be open, or stopped.
Open pipes have a hole in the top of the pipe.
In the picture, the first frame contains open flue pipes and the second shows reed pipes , which are always open at the top. Stopped pipes have a stopper in the top seen as the dark stick on the top of the wooden pipes in the third frame of the picture. Stopped metal pipes can be identified by the metal cap and felt band at the top of the pipe. The last four rows of pipes in the fourth frame are stopped metal pipes. Metal pipes can also be tapered.