Augmented undecime

The augmented eleventh (A11) spans 18 semitones. It is the compound form of the augmented fourth and a defining color in modern extended harmony.

Construction and spelling

A11 is built as an octave plus augmented fourth, for example C-F# above the octave. It may share pitch space with d12 in equal temperament, but spelling carries different function. This distinction is central in harmonic interpretation.

Harmonic and melodic usage

Harmonically, A11 appears in Lydian-oriented major sounds, dominant alterations, and color-rich voicings. Melodically, it brings bright tension and lift. It is widely used for vivid upper-extension color.

Examples

• #11 color on major and dominant sonorities
• Lydian textures with raised fourth identity
• Upper-structure voicings emphasizing brightness

In practice

Practice A11 against P11 and d12 to separate functionally distinct spellings. Voice #11 carefully against chord tones to manage tension. This improves extension clarity and arranging precision.