In music theory, an eleventh is a compound interval consisting of an octave plus a fourth.

A perfect eleventh spans 17 and the augmented eleventh 18 semitones, or 10 steps in a diatonic scale.

Since there are only seven degrees in a diatonic scale, the eleventh degree is the same as the subdominant (IV). The eleventh is considered highly dissonant with the major third.

An eleventh chord is the stacking of five thirds in the span of an eleventh. In common practice tonality, it usually had subdominant function as minor eleventh chord on the second degree (supertonic) of the major scale.