Constant structure

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A scale is said to have constant structure (CS) if its generic interval classes are distinct. That is, each interval occurs always subtended by the same number of steps. This means that you never get something like an interval being counted as a fourth one place, and a fifth another place.

The term "constant structure" was coined by Erv Wilson. In academic music theory, constant structure is called the partitioning property, but Erv got there first.

To determine if a scale is CS, all possible intervals between scale steps must be evaluated. An easy way to do this is with an interval matrix (Scala can do this for you). A CS scale will never have the same interval appear in multiple columns of the matrix (columns correspond to generic interval classes).

Examples

This common pentatonic scale is a constant structure: 1/1 - 9/8 - 5/4 - 3/2 - 5/3 - 2/1

Here is the interval matrix of this scale:

1 2 3 4 5 (6)
1/1 1/1 9/8 5/4 3/2 5/3 2/1
9/8 1/1 10/9 4/3 40/27 16/9 2/1
5/4 1/1 6/5 4/3 8/5 9/5 2/1
3/2 1/1 10/9 4/3 3/2 5/3 2/1
5/3 1/1 6/5 27/20 3/2 9/5 2/1

Note that every interval always appears in the same position (column). For example, 3/2, which happens to appear three times, is always the "fourth" of this scale - never the "third" or "fifth".

This pentatonic scale is not a constant structure: 1/1 - 25/24 - 6/5 - 3/2 - 5/3 - 2/1

Its interval matrix:

1 2 3 4 5 (6)
1/1 1/1 25/24 6/5 3/2 5/3 2/1
25/24 1/1 144/125 36/25 8/5 48/25 2/1
6/5 1/1 5/4 25/18 5/3 125/72 2/1
3/2 1/1 10/9 4/3 25/18 8/5 2/1
5/3 1/1 6/5 5/4 36/25 9/5 2/1

Note the highlighted intervals that occur in more than one column. For example, 5/4 may occur as both the "second" and "third" steps of the scale. Thus, this scale does not have constant structure.

Another example of a familiar scale that is not CS is the 7-note diatonic scale in 12edo.

Interval matrix as steps of 12edo:

1 2 3 4 5 6 7 (8)
0 0 2 4 5 7 9 11 12
2 0 2 3 5 7 9 10 12
4 0 1 3 5 7 8 10 12
7 0 2 4 6 7 9 11 12
9 0 2 4 5 7 9 10 12
11 0 2 3 5 7 8 10 12
12 0 1 3 5 6 8 10 12

Interval matrix as note names:

1 2 3 4 5 6 7 (8)
C C D E F G A B C
D C D Eb F G A Bb C
E C Db Eb F G Ab Bb C
F C D E F# G A B C
G C D E F G A Bb C
A C D Eb F G Ab Bb C
B C Db Eb F Gb Ab Bb C

F# and Gb are the same pitch (600 cents) in 12edo, and this interval occurs as both an (augmented) fourth and a (diminished) fifth - so not constant structure. (However, a meantone tuning of this scale, in which F# and Gb are distinguished, would have constant structure.)

Density of CS Scales in EDOs

EDO Number of CS Scales Percent of Scales CS Corresponding Fraction
1 1 100.0% 1/1
2 1 100.0% 1/1
3 2 100.0% 1/1
4 2 66.7% 2/3
5 5 83.3% 5/6
6 4 44.4% 4/9
7 11 61.1% 11/18
8 11 36.7% 11/30
9 22 39.3% 11/28
10 20 20.2% 20/99
11 45 24.2% 15/62
12 47 14.0% 47/335
13 85 13.5% 17/126
14 88 7.6% 88/1161
15 163 7.5% 163/2182
16 165 4.0% 11/272
17 294 3.8% 49/1285
18 313 2.2% 313/14532
19 534 1.9% 89/4599
20 541 1.0% 541/52377

See also