Unit of length for measuring vibrating air‐column, and therefore a measure of pitch. An air‐column of 8′ vibrates at twice the speed of a 16′ column, and thus emits a note an octave higher. Org. stops are classified by the sound which will be emitted if the note representing 8′ C is struck. An 8′ stop sounds the note itself, 16′ the note an octave below, 32′ the note two octaves below. A 4′ stop sounds the note an octave above, 2′ two octaves above, 1′ three octaves above.