Page "Organ stop" Paragraph 21

Mutations usually sound at pitches in the harmonic series of the fundamental and, except where they are derived from unit ranks, are always tuned pure.

In some organs, lower pitches are used to create difference tones e. g., Quintbass 10 '.

Such ' helper ranks " that sound at the fifth just above or fourth below the fundamental ( e. g., Bourdon 16 '), can create the impression of a stop an octave lower than the fundamental ( i. e., Bourdon 32 '), saving the space and money otherwise needed for larger bass pipes.