pipes are arranged by timbre and pitch into ranks. a rank is a set of pipes of the same timbre but multiple pitches (one for each note on the keyboard), which is mounted (usually vertically) onto a windchest. the stop mechanism admits air to each rank. for a given pipe to sound, the stop governing the pipe's rank must be engaged, and the key corresponding to its pitch must be depressed. ranks of pipes are organized into groups called divisions. each division generally is played from its own keyboard and conceptually comprises an individual instrument within the organ.