There is a separate pipe for each key in every tone color. The pipes are arranged in rows or "ranks," according to these tone colors. To bring a rank of pipes into play, the organist pulls a knob or operates a tablet called a "stop." (This term comes from the fact that pushing a knob in "stops" the pipes from speaking.) Some stops control two or more ranks, which blend to make one tone color. Stops may also be called "voices" or "registers." Each stop on an organ will have a number that tells the pitch at which it sounds. A stop that sounds the written pitch is labeled 8'. (This refers to the speaking length of an open pipe at low C, the lowest written pitch on the organ. All stops that sound the written pitch are called eight-foot stops, even if their low Cs are not actually eight feet long; stops of 4' pitch sound an octave higher than written; 2' stops sound two octaves higher, etc. As you might expect, 16' stops sound an octave lower than written and 32' stops sound two octaves lower.