The Harmonic Series

4' PITCH (the second partial)

As the string vibrates as a whole unit, it also vibrates in halves, producing the octave, one octave above tenor C (middle C). This is the second partial or harmonic, and the first overtone.

If you were to play tenor C at the organ and pull a 4-foot stop only, you would hear the octave, one octave above tenor C (middle C).

4' stops are octave-sounding stops, which sound the note at some octave above or below the key being played.