The Harmonic Series

16' PITCH (one octave below the fundamental)

Doubling the length of the vibrating tenor-C string produces a pitch one octave below tenor C.

If you were to play tenor C at the organ and pull a 16-foot stop only, you would hear the C one octave below tenor C.

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