The New International Encyclopædia/Harmonic Stop
HARMONIC STOP. An organ-stop, having pipes double the usual length, and pierced mid-way, so that the tone produced is an octave higher than the ordinary pitch. Harmonic stops are composed generally of more than a single rank of pipes, tuned in octaves, double octaves, and double or triple thirds and fifths above the natural pitch of the keys; they comprise the mixture, furniture, cornet, etc. Those which have only a single rank of pipes tuned in thirds, fifths, with their octaves above the pitch represented on the keyboard, are called ‘mutation stops.’ They were introduced to give additional power to the ‘foundation stops,’ and also to produce a more brilliant effect in the performance of certain styles of music. See Organ.