Regular diatonic tuning

A regular diatonic tuning is any musical scale consisting of "tones" and "semitones" arranged in any rotation of the sequence TTSTTTS which adds up to the octave with all the T's being the same size and all the S's the being the same size, with the 'S's being smaller than the 'T's. In such a tuning, then the notes are connected together in a chain of seven fifths, all the same size which makes it a Linear temperament with the tempered fifth as a generator.

Overview

For the ordinary diatonic scales described here, the -s are tones and the -s are semitones which are half, or approximately half the size of the tone. But in the more general regular diatonic tunings, the two steps can be of any relation within the range between and in musical cents. Note that regular diatonic tunings are not limited to the notes of any particular diatonic scale used to describe them.
One may determine the corresponding cents of,, and the fifth, given one of the values:
When the semitones,, are reduced to zero the octave is or a five tone equal temperament. As the semitones get larger, eventually the steps are all the same size, and the result is in seven tone equal temperament These two extremes are not included as "regular" diatonic tunings, because to be "regular" the pattern of five large and two small steps has to be preserved; everything in between is regular, however small the semitones are without vanishing completely, or however large they become while still being strictly smaller than a whole tone.
"Regular" here is understood in the sense of a mapping from Pythagorean diatone such that all the interval relationships are preserved.
For instance, in all regular diatonic tunings, just as for the Pythagorean diatonic:

The notes are connected together through a chain of six fifths reduced to the octave, or equivalently, through ascending fifths and descending fourths.
A chain of three tones spaced in equal-sized fifths generates a whole tone.
A sequence of six tones spaced in fourths generates a semitone in the same way.
A sequence of five tones spaced in fifths generates a major third, consisting of two whole tones.
A chain of four tones spaced in fourths generates a minor third

and so on; in all those examples the result is "reduced to the octave".
If one breaks the rule for "regular" that must be smaller than and continues to increase the size of further, so that it becomes larger than the, one gets irregular scales with two large steps and five small steps, and eventually, when all the -s vanish the result is, so a division of the octave into tritones. However, these strange scales are only mentioned here to dismiss them; they not regular diatonic tunings.
All regular diatonic tunings are also linear temperaments, i.e. regular temperaments with two generators: the octave and the tempered fifth. One can use the tempered fourth as an alternative generator, but the tempered fifth is the more usual choice, and in any case, because fifths and fourths are octave complements, rising by perfect fourths produces the same result as rising by fifths.
All regular diatonic tunings are also generated collections and the chain of fifths can be continued in either direction to obtain a twelve tone system ' ' ' ', where the interval '-' is the same as '-', etc., another moment of symmetry with two interval sizes.
Instead of there being one semitone,, there are actually two: the chromatic semitone,, and the diatonic semitone, ; is another name for. Three notes spaced by a chromatic and diatonic semitone make a whole tone between the first and the last: The small difference in pitch between the two is called a comma, usually prefixed by the name of the tuning system that generates it, such as a syntonic comma, or Pythagorean comma, or a 53 comma.
A chain of eight notes spaced in fifths generates a chromatic semitone,, as the space between the first and the last; it is the change of pitch needed to raise a minor tone to a major tone; for instance from ' to '. For any tuning, the chromatic semitone is the space between a flat note and its natural, or a natural note and its sharp; between a white key and either the black key above it or the black key below it ; in most tunings, the two intervals are different. The diatonic semitone,, called above, is the change in pitch of a sequence of six notes spaced by fifths, e.g. from ' to ' or ' to '. For any tuning, the diatonic semitone is the relative pitch difference on a standard keyboard between two white keys that have no black key between them. The pattern of chromatic and diatonic semitones is or some mixed-around version of it. Here, the seven equal system is the limit as the chromatic semitone tends to zero, and the five tone system in the limit as the diatonic semitone tends to zero.

Range of recognizability

The regular diatonic tunings include all linear temperaments within Easley Blackwood's "Range of Recognizability" in his The Structure of Recognizable Diatonic Tunings for diatonic tunings with

the fifth tempered to between 4/7 and 3/5 of an octave;
the major and minor seconds both positive;
the major second larger than the minor second.

However, his "range of recognizability" is more restrictive than "regular diatonic tuning". For instance, he requires the diatonic semitone to be at least 25 cents in size. See for a summary.

Significant regions within the range

When the fifths are slightly flatter than in just intonation, then we are in the region of the historical meantone tunings, which distribute or temper out the syntonic comma. They include:

12 tone equal temperament, practically indistinguishable from meantone
19 tone equal temperament – equivalent to meantone; almost exactly achieves minor thirds
31 tone equal temperament – equivalent to meantone; achieves major thirds extremely close to ; fifth is 696.77 cents; augmented 6th is within 1 cent of
43 tone equal temperament – equivalent to meantone – achieves pure major sevenths of almost exactly ; fifth is 697.67 cents
55 tone equal temperament – equivalent to meantone – achieves a rational diatonic fifth is 698.18 cents

When the fifths are exactly, or around 702 cents, the result is the Pythagorean diatonic tuning.
For fifths slightly narrower than, the result is a schismatic temperament, where the temperament is measured in terms of a fraction of a schisma - the amount by which a chain of eight fifths reduced to an octave is sharper than the just minor So for instance, a temperament will achieve a pure in an ascending chain of eight fifths. 53 tone equal temperament achieves a good approximation to schismatic temperament.
If the fifth is tuned slightly sharp of just, between 702.4 and 705.9 cents, the result is very sharp major thirds with ratios near and very flat minor thirds around . These tunings are known as "parapythagorean" tunings.
At 705.882 cents, with fifths tempered in the wide direction by 3.929 cents, the result is the diatonic scale in 17 tone equal temperament. Beyond this point, the regular major and minor thirds approximate simple ratios of numbers with prime factors 2-3-7, such as the or septimal major third and or septimal minor third. At the same time, the regular tones more and more closely approximate a large tone, and regular minor sevenths the "harmonic seventh" at the simple ratio of . This septimal range, known as "superpythagorean", extends out to around 711.11 cents or 27 tone equal temperament, or a bit further.
That leaves the two extremes:

The "inframeantone" or "flattone" range is the flatmost extreme, where the fifth is between the lower bound for the regular diatonic of 7 tone equal temperament and the range of historical meantones beginning around 19 tone equal temperament. Here, the diatonic semitones approach the size of the whole tone.
* The range between 690.91 cents and 685.71 cents has been called the "deeptone" range by some.
The "ultraseptimal" or "ultrapythagorean" range encompasses the sharpmost extreme, between 711.11 cents as seen in 27 tone equal temperament all the way to the upper bound of the regular diatonic at 720 cents or 5 tone equal temperament. As one tends towards 5 equal, the diatonic semitones become smaller and smaller.

Diatonic scales constructed in equal temperaments can have fifths either wider or narrower than a Here are a few examples:

15, 17, 22, and 27 have fifths wider than a just
12, 19, 31, and 43 have fifths narrower than a

Syntonic temperament and timbre

The term syntonic temperament describes the combination of

the continuum of tunings in which the tempered perfect fifth is the generator and the octave is the period;
Comma sequences that start with the syntonic comma ; and
the "tuning range" of P5 temperings in which the generated minor second is neither larger than the generated major second, nor smaller than the unison.

This combination is necessary and sufficient to define a set of relationships among tonal intervals that is invariant across the syntonic temperament's tuning range. Hence, it also defines an invariant mapping -- all across the tuning continuum -- between the notes at these generated tonal intervals, and the corresponding partials of a similarly-generated pseudo-Harmonic timbre. Hence, the relationship between the syntonic temperament and its note-aligned timbres can be seen as a generalization of the special relationship between Just Intonation and the Harmonic Series.
Maintaining an invariant mapping between notes and partials, across the entire tuning range, enables Dynamic tonality, a novel expansion of the framework of tonality, which includes timbre effects such as primeness, conicality, and richness, and tonal effects such as polyphonic tuning bends and dynamic tuning progressions.
If one considers the syntonic temperament's tuning continuum as a string, and individual tunings as beads on that string, then one can view much of the traditional microtonal literature as being focused on the differences among the beads, whereas the syntonic temperament can be viewed as being focused on the commonality along the string.
[Image:Rank-2 temperaments with the generator close to a fifth and period an octave.jpg|right|250px|thumb|Figure 1: The syntonic temperament's tuning continuum, from (Milne et al. 2007)]
The notes of the syntonic temperament are best played using the Wicki-Hayden note layout. Because the syntonic temperament and the Wicki-Hayden note-layout are generated using the same generator and period, they are isomorphic with each other; hence, the Wicki-Hayden note-layout is an isomorphic keyboard for the syntonic temperament. The fingering-pattern of any given musical structure is the same in any tuning on the syntonic temperament's tuning continuum. The combination of an isomorphic keyboard and continuously variable tuning supports Dynamic tonality as described above.
As shown in the figure at right, the tonally valid tuning range of the syntonic temperament includes a number of historically important tunings, such as the currently popular 12-tone equal division of the octave, the meantone tunings, and Pythagorean tuning. Tunings in the syntonic temperament can be equal, non-equal, circulating, and Just.
[Image:Syntonic pitch changes.jpg|left|thumb|250px|Figure 2: Change in widths of intervals of the syntonic temperament across its tuning continuum (tonic is D)]
The legend of Figure 2 shows a stack of P5s centered on D. Each resulting note represents an interval in the syntonic temperament with D as the tonic. The body of the figure shows how the widths of these intervals change as the width of the P5 is changed across the syntonic temperament's tuning continuum.

At P5 ≈ 685.7 cents, the intervals converge on just 7 widths, producing 7-edo. S/T = 0.
At P5 ≈ 694.7, the gaps between these 19 intervals are all equal, producing 19-edo tuning. S/T = 2/3.
At P5 ≈ 696.8, a stack of 31 such intervals would show equal gaps between each such interval, producing 31-edo tuning. S/T = 3/5.
At P5 = 700.0, the sharp notes and flat notes are equal, producing 12-edo tuning. S/T = 1/2.
At P5 ≈ 701.9, a stack of 53 such intervals - each just 3/44 of a cent short of a pure fifth - makes 31 octaves, producing 53-edo tuning. S/T = 4/9.
etc....
at P5 = 720.0 cents, the pitches converge on just 5 widths, producing 5-edo. S/T = 1.

Research projects regarding the syntonic temperament

The research program Musica Facta investigates the musical theory of the syntonic temperament.
The music theory of the is based on the syntonic temperament. Guido 2.0 seeks to achieve a 10x increase in the efficiency of music education by exposing the invariant properties of music's syntonic temperament with geometric invariance. Guido 2.0 is the Music Education aspect of Musica Facta.