21/11
| Interval information |
undecimal major seventh,
pentacircle major seventh
(Shannon, [math]\sqrt{nd}[/math])
[sound info]
21/11, commonly known as the large undecimal diminished octave, is an 11-limit interval, and the octave complement of 22/21.
In many notation systems (e.g. FJS, HEJI), it is an imperfect octave, as it is the octave minus a stack consisting of an undecimal quartertone (33/32) and a septimal comma (64/63), neither of which changes the scale degree or quality. However, it is only sharp of the Pythagorean major seventh (243/128) by a pentacircle comma (896/891). For this reason it could be called the pentacircle major seventh.
Of note is that the Huygens-Fokker Foundation dubs this interval the undecimal major seventh, which also makes sense, resulting in this interval being perhaps best classified as a "sevtave" – a type of cross between a seventh and an octave.
See also
- 22/21 – its octave complement