Pseudo-octave
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A pseudo-octave is an interval whose frequency ratio is not 2:1, the definition of an octave, but is treated in some way or ways equivalent to this ratio. One example being the stretched octave: 2.1:1, which sounds out of tune played with harmonic overtones, but sounds strange but in tune when played with tones whose overtones are stretched equivalently, while the 2:1 octave then sounds out of tune. Stretched octaves are most commonly found on the piano, where the inharmonicity caused by string thickness and tension makes it necessary to widen ever interval very slightly. The octaves of Balinese gamelans are never tuned 2:1, but instead are stretched or compressed in a consistent manner throughout the range of each individual gamelan. Another example is the tritave of the Bohlen-Pierce scale.
Other common names for the pseudo-octave are the Interval of Equivalence (IoE), the Repeat Ratio, and the nonoctave.