2.3.7 subgroup
The 2.3.7 subgroup[1] (za in color notation) is a just intonation subgroup consisting of rational intervals where 2, 3, and 7 are the only allowable prime factors, so that every such interval may be written as a ratio of integers which are products of 2, 3, and 7. This is an infinite set and still infinite even if we restrict consideration to a single octave. Some examples within the octave include 3/2, 7/4, 7/6, 9/7, 9/8, 21/16, and so on.
The 2.3.7 subgroup is a retraction of the 7-limit, obtained by removing prime 5. Its simplest expansion is the 2.3.7.11 subgroup, which adds prime 11.
A notable subset of the 2.3.7 subgroup is the 1.3.7 tonality diamond, comprised of all intervals in which 1, 3 and 7 are the only allowable odd numbers, once all powers of 2 are removed, either for the intervals of the scale or the ratios between successive or simultaneously sounding notes of the composition. The complete list of intervals in the 1.3.7 tonality diamond within the octave is 1/1, 8/7, 7/6, 4/3, 3/2, 12/7, 7/4, and 2/1.
Another such subset is the 1.3.7.9 tonality diamond, which adds the following intervals to the previous list: 9/8, 9/7, 14/9, and 16/9.
When octave equivalence is assumed, an interval can be taken as representing that interval in every possible voicing. This leaves primes 3 and 7, which can be represented in a 2-dimensional lattice diagram, each prime represented by a different dimension, such that each point on the lattice represents a different interval class.
Scales
- Zo minor pentatonic: 1/1 7/6 4/3 3/2 7/4 2/1
- Ru pentatonic: 1/1 9/8 9/7 3/2 12/7 2/1
- Zo in: 1/1 9/8 7/6 3/2 14/9 2/1
- Zo minor: 1/1 9/8 7/6 4/3 3/2 14/9 7/4 2/1
- Za harmonic minor: 1/1 9/8 7/6 4/3 3/2 14/9 27/14 2/1 (zo minor with a ru 7th)
- Za diasem/Tas[9] (left-handed): 1/1 9/8 7/6 21/16 4/3 3/2 14/9 7/4 16/9 2/1
Regular temperaments
Rank-1 temperaments (edos)
A list of edos with progressively better tunings for the 2.3.7 subgroup: 5, 12, 14, 17, 22, 31, 36, 77, 94, 130, 135, 171, 265, 306, 400, 571, 706, 1277 and so on.
Another list of edos which provides relatively good tunings for the 2.3.7 subgroup (relative error < 2.5%): 36, 41, 77, 94, 99, 130, 135, 171, 207, 229, 265, 301, 306, 364, 400, 436, 441, 477, 494, 535, 571, 576, 607, 648, 665, 670, 701, 706, 742, 747, 783, 836, 841, 877, 913, 935, 971, 976, 1007, 1012, 1048, 1106, 1147, 1178, 1183, 1236, 1241, 1277 and so on.
Rank-2 temperaments
Music
- From Ancient Worlds (for harmonic piano), 1992
- Revelation: Music in Pure Intonation, 2007
Notes
- ↑ Sometimes incorrectly named 2.3.7-limit or 2.3.7-prime limit; a prime limit is a subgroup spanned by all primes up to a given prime, and "limit" used alone usually implies prime limit.