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Ninth In music, a ninth is a compound interval consisting of an octave plus a second. Like the second, the interval of a ninth is classified as a dissonance in common practice tonality. Since a ninth is an octave larger than a second, its sonority level is considered less dense. 
Wikipedia
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Nineth (adjective) alternative spelling of ninth 
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Ninth (adjective) The ordinal form of the number nine. 
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Ninth (noun) The person or thing in the ninth position. 
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Ninth (noun) One of nine equal parts of a whole. 
Wiktionary
