to facilitate a large range of timbres, organ stops exist at different pitch levels. a stop that sounds at unison pitch when a key is depressed is referred to as being at 8′ (pronounced "eight-foot") pitch. this refers to the length of the lowest-sounding pipe in that rank, which is approximately . for the same reason, a stop that sounds an octave higher is at 4′ pitch, and one that sounds two octaves higher is at 2′ pitch. likewise, a stop that sounds an octave lower than unison pitch is at 16′ pitch, and one that sounds two octaves lower is at 32′ pitch. stops of different pitch levels are designed to be played simultaneously.