An n-ary scale is a scale with exactly n distinct step sizes; a scale's arity is the number of distinct step sizes it has. Unary, binary and ternary scales are scales with exactly 1, 2 and 3 step sizes, respectively.
Unary scales are equal tunings. The class of binary scales consists of all MOS scales and every alteration-by-permutation of a MOS scale, but do not include altered MOS scales such as the harmonic minor scale, msmmsLs, which gain additional step sizes from the alteration. Ternary scales are much less well-understood than binary ones, but one well-studied type of ternary scales is the class of generator-offset scales. Most known facts about ternary scales on the wiki can be found on the page rank-3 scale (which is mostly about specifically ternary scales).
History of the term
The terms binary and ternary are already used in some academic literature in reference to words over an alphabet, in particular to circular words that represent abstract scales; see e.g. Bulgakova, Buzhinsky and Goncharov (2023), "On balanced and abelian properties of circular words over a ternary alphabet".
Difference from scale rank
Certain abstract scale theorists in the xen community have taken to using the n-ary terminology in view of the subtlety of the notion of a scale's rank. Examples of this subtlety are:
- Equal tunings contain MOS scales and ternary scales, but the group generated by the step sizes in these tunings of the scales must be rank 1.
- Certain chroma-altered MOS scales, which are contained in the group generated by the period and the generator of the unaltered MOS are ternary. An example is harmonic minor in any non-edo diatonic tuning, a chroma-alteration of the diatonic MOS with step pattern msmmsLs.
The term n-ary disregards the rank of the group generated by the step sizes, although an n-ary scale is still generically rank-n (the group generated by the n step sizes Xi > 0, i = 1, ..., n, has rank n, not lower, for almost all choices of Xi, in the same sense that almost all real numbers between 0 and 1 are irrational).
List of named ternary scales
The following is a list of (temperament-agnostic) names that have been given to ternary scales.
- diachromedye (5L 2M 5S)
- whitedye (5L 2M 7S)