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organ tone production
From the earliest times, organs have, therefore, been supplied with what are known generically as mixture stops, which have several high-pitched pipes to each note. But, since, for example, a 1-foot rank could not be carried right up to the top note, it breaks back an octave at some convenient point in the compass. Ranks pitched even higher will break back more than once. Thus, in the bass, a...
...consist of pipes sounding higher ( e.g., by five notes) than the other pipes, rather than in unison with them. Used in combination with unison pipes they add an incisive quality to the sound. Mixture stops consist of two or more ranks of pipes, both unison and mutation ranks, controlled by a single stop.