Auxiliary stops (mutation stops)
Organ stops that sound not the fundamental but fifths, thirds, or mixtures and reinforce overtones.
What it means
Auxiliary stops (Hilfsstimmen) are organ stops that sound not the played note in octaves but other overtones, such as fifths, thirds, or several pipes as a mixture. They reinforce overtones of the basic sound and give it brilliance and sharpness. They are also called mutation stops or aliquots.
Musical use
Auxiliary stops are used not alone but together with the foundation stops. A fifth at 2⅔ʼ makes the note a twelfth above every played note sound, a tierce at 1⅗ʼ the third in a higher octave. Together with the basic sound they yield tone colors such as cornet or sesquialtera, with which solo lines can be brought out. In the mixture several pipes in fifths and octaves are united into a brilliant crown of the sound.
Auxiliary stops should be distinguished from foundation stops, which stand in octaves to the played note, and from reed stops, which sound with reeds instead of flue pipes. Played alone, auxiliary stops sound unnatural; only with the foundation stops do they form a meaningful sound.
Performance practice
Register auxiliary stops carefully: begin with a group of foundation stops and add fifth or mixture gradually until the desired brilliance is reached. Check the balance in the room, since overtone stops carry far and quickly become cutting.