# Just intonation subgroup

A just intonation subgroup is a group generated by a finite set of positive rational numbers via arbitrary multiplications and divisions. Using subgroups implies a way to organize just intonation intervals such that they form a lattice. Therefore it is closely related to regular temperament theory.

Just intonation subgroups can be described by listing their generators with full stops between them; we use said convention below. In standard mathematical notation, let c1, ..., cr be positive reals, and suppose vk is the musical interval of log2(ck) octaves. Then

[math]c_1.c_2.\cdots.c_r := \operatorname{span}_\mathbb{Z} \{v_1, ..., v_k\}.[/math]

There are three categories of subgroups:

• Prime subgroups (e.g. 2.3.7) contain only primes
• Composite subgroups (e.g. 2.9.5) contain composite numbers and perhaps prime numbers too
• Fractional subgroups (e.g. 2.3.7/5) contain fractional numbers and perhaps prime and/or composite numbers too

For composite and fractional subgroups, not all combinations of numbers are mathematically valid bases for subgroups. For example, 2.3.9 has a redundant generator 9, and both 2.3.15 and 2.3.5/3 can be simplified to 2.3.5.

A prime subgroup that does not omit any primes < p (e.g. 2.3.5, 2.3.5.7, 2.3.5.7.11, etc. but not 2.3.7 or 3.5.7) is simply called p-limit JI. It is customary of just intonation subgroups to refer only to prime subgroups that do omit such primes, as well as the other two categories.

The following terminology has been proposed for streamlining pedagogy: Given a subgroup written as generated by a fixed (non-redundant) set: a.b.c.[…].d, call any member of this set a basis element, structural prime, or formal prime.[1] For example, if the group is written 2.5/3.7/3, the basis elements are 2, 5/3 and 7/3.

## Normalization

A canonical naming system for just intonation subgroups is to give a normal interval list for the generators of the group, which will also show the rank of the group by the number of generators in the list (the Hermite normal form should be used here, not the canonical form, because in the case of subgroups, enfactoring is sometimes desirable, such as in the subgroup 2.9.7 which should not be reduced to 2.3.7 by subgroup canonicalization). Below we give some of the more interesting subgroup systems. If a scale is given with the system, it means the subgroup is generated by the notes of the scale.

## Index

Intuitively speaking, the index measures the relative size of the subgroup within another subgroup, which is usually the p-limit.

Subgroups in the strict sense come in two flavors: finite index and infinite index. For example, the subgroups generated by 4 and 3, by 2 and 9, and by 4 and 6 all have index 2 in the full 3-limit (Pythagorean) group. Half of the 3-limit intervals will belong to any one of them, and half will not, and all three groups are distinct. On the other hand, the group generated by 2, 3, and 7 is of infinite index in the full 7-limit group, which is generated by 2, 3, 5 and 7. The index can be computed by taking the determinant of the subgroup basis matrix, whose columns are the monzos of the generators.

## Generalization

Non-JI intervals can also be used as basis elements, when the subgroup in question contains non-JI intervals. For example, 2.sqrt(3/2) (sometimes written 2.2ed3/2) is the group generated by 2/1 and 350.978 cents, the square root of 3/2 (a neutral third which is exactly one half of 3/2). This is closely related to the 3L 4s mos tuning with neutral third generator sqrt(3/2).

## List of selected subgroups

### 7-limit subgroups

2.3.7
• EDOs5, 17, 31, 36, 135, 571
• Archytas Diatonic [8/7, 32/27, 4/3, 3/2, 12/7, 16/9, 2/1]
• Safi al-Din Septimal [8/7, 9/7, 4/3, 32/21, 12/7, 16/9, 2/1]
2.5.7
2.3.7/5
2.5/3.7
2.5.7/3
2.5/3.7/3
2.27/25.7/3
• EDOs9
• In effect, equivalent to 9EDO, which has a 7-limit version given by [27/25, 7/6, 63/50, 49/36, 72/49, 100/63, 12/7, 50/27, 2]
2.9/5.9/7
3.5.7
• Does not have octaves, commonly used for non-octave EDTs

2.3.11
2.5.11
2.7.11
2.3.5.11
2.3.7.11
2.5.7.11
2.5/3.7/3.11/3

### 13-limit subgroups

2.3.13
2.3.5.13
2.3.7.13
• EDOs10, 26, 27, 36, 77, 94, 104, 130, 234
• Buzurg [14/13, 16/13, 4/3, 56/39, 3/2]
• Safi al-Din tuning [8/7, 16/13, 4/3, 32/21, 64/39, 16/9, 2/1]
• Ibn Sina tuning [14/13, 7/6, 4/3, 3/2, 21/13, 7/4, 2]
2.5.7.13
2.5.7.11.13
2.3.13/5
2.3.11/5.13/5
2.3.11/7.13/7
2.7/5.11/5.13/5