In an equal-step tuning, the distance between adjacent steps is of constant size. The size of this single step is given explicitly (e.g. 88 cent equal temperament) or as a fraction of a larger interval (e.g. 13 equal tones per octave). Any interval, rational, Just, or irrational, may be used as the basis for an equal tuning, although divisions of the octave are most common leading to EDO systems. When a just interval is equally divided, it is assumed none of the resulting intervals are just, because if the interval has a rational root it is seen as a division of that root.
When a tuning is called "n-tone equal temperament" (abbreviated n-tET or n-ET), this usually means "n divisions of 2/1, the octave, or some approximation thereof" but it also implies a mindset of temperament—that is, of a harmony-centric, JI-approximation-based understanding of the scale. If you are wondering how equal divisions of the octave can become associated with temperaments, the page EDOs to ETs may help clarify.
There are many reasons why one might choose to not consider JI approximations when dealing with equal tunings, and thus not treat equal tunings as temperaments. In such case, the less theory-laden term EDO(occasionally written ED2), meaning "equal divisions of the octave" (or "equal divisions of 2/1"), leaves comparison to JI out of the picture, aside from the octave itself (which is assumed to be Just). There are other less standard terms, many in the Tonalsoft Encyclopedia. More generally, the term EDn can be used, where n is any harmonic of the harmonic series. For example, the equal-tempered Bohlen-Pierce scale may also be referred to as 13-ED3, for 13 equal divisions of 3/1 (the 3rd harmonic).
As the steps are tuned to be equal, equal scales may be taken to close anywhere composers wish them to. Barring the convention of closing equal divisions of particular just intervals at those stated just intervals, there are infinite synonymous names for each equal scale. Barring further the large number of names which would be avoided in discourses on comparative modality and tonality, there is still a a great width to the universe of modes and keys which modal and tonal compositional art can access.
As there are infinite intervals, there are infinite equal scales. Barring technicalities, there are large quantities of perceivably different equal scales. Seeing such a diverse menagerie at their disposal, some composers choose to combine multiple equal tunings sequentially or simultaneously.
Simultaneous equal divisions
What do 12ed2, 19ed3, and 28ed5 all have in common? They're all approximately the same scale. This happens because 12ed2 is an accurate temperament (for its size) that contains relatively close approximations of 3/1 and 5/1. In contrast, 11ed2 does not correspond closely to any equal division of 3/1 or 5/1.
The following plot shows equal divisions of 2/1, 3/1, 5/1, and 7/1, and points out some instances when three or more of them happen to be close together. Note that any equal division of 2/1 is automatically an equal division of 4/1; and if something is simultaneously a good equal division of both 2/1 and 3/1, then it's a good equal division of 6/1 as well.
(Unlimited resolution version: equal.svg)
For the mathematically inclined, this kind of diagram is closely related to the Riemann zeta function.
Gallery of equal divisions
- ...of the Octave/Duple (2/1)
- by far the most widespread ones
- …of ninths (e. g. 9/4)
- …of tenths (e. g. 7/3)
- ...of the Tritave/Triple (3/1)
- most famously the BP scale, but lots of others, too
- ...of the Double Octave (4/1)
- ...of the Just Major 17th (5/1)
- …of the hexatave (6/1)
- ...of the 7th Natural (7/1)
- ... of the Triple Octave (8/1)
most famously Carlos Alpha, Beta and Gamma, but lots of others, too
...of the Perfect Fourth (4/3)
3 - Cube Root of 4/3
9 - 'Noleta' Scale
...of the Tridecimal Ultramajor Third (13/10)
...of a given cents value
...of various whole tones
- 9_8ths_equal_temperament - multiples of 9/8
- 10_9ths_equal_temperament - multiples of 10/9
- 12_11th_equal_temperament - multiples of 12/11
- 13_12ths_equal_temperament - multiples of 13/12