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In music or music theory an eleventh play is the note eleven scale degrees from the root of a chord and also the interval between the root and the eleventh. The interval can be also described as a compound fourth, spanning an octave plus a fourth.
Since there are only seven degrees in a diatonic scale the eleventh degree is the same as the subdominant.
The eleventh is considered highly dissonant with the third.
A perfect eleventh is an eleventh which spans exactly 17 semitones. It can be also described as a compound perfect fourth, spanning an octave plus a perfect fourth.
References
Eleventh Wikipedia(Text) CC BY-SA