The Dominant Eleventh Chord adds a perfect eleventh (an octave plus a perfect fourth) to the structure of the Dominant 9. The theoretical notes are: root, major third, perfect fifth, minor seventh, major ninth, and perfect eleventh. However, the perfect eleventh forms a semitone interval with the major third of the chord, creating a strong dissonance that is almost always undesirable or perceived as 'muddy'. For this reason, the perfect eleventh is almost always omitted or, more commonly, altered to an augmented eleventh (#11). If the natural eleventh is present, the major third is often omitted to accommodate it, giving the chord a 'suspended' or 'bluesy' sound, but with the risk of sounding 'muddy' if all voices are not managed with extreme care. The 'natural' Dominant 11 chord is less common than the Dominant 9 and 13 precisely because of this potential harmonic collision. When used, it often suggests a less defined modal or blues atmosphere.