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In music, 31 equal temperament, 31 ET, which can also be abbreviated 31 TET (31 tone ET) or 31 EDO (equal division of the octave), also known as tricesimoprimal, is the tempered scale derived by dividing the octave into 31 equally-proportioned steps (equal frequency ratios). Play Each step represents a frequency ratio of √2 , or 38.71 cents (Play).

31 EDO is a very good approximation of quarter-comma meantone temperament. More generally, it is a regular diatonic tuning in which the tempered perfect fifth is equal to 696.77 cents, as shown in Figure 1. On an isomorphic keyboard, the fingering of music composed in 31 EDO is precisely the same as it is in any other syntonic tuning (such as 12 EDO), so long as the notes are spelled properly—that is, with no assumption of enharmonicity.

History and use

Division of the octave into 31 steps arose naturally out of Renaissance music theory; the lesser diesis‍ – the ratio of an octave to three major thirds, 128:125 or 41.06 cents – was approximately one-fifth of a tone or two-fifths of a semitone. In 1555, Nicola Vicentino proposed an extended-meantone tuning of 31 tones. In 1666, Lemme Rossi first proposed an equal temperament of this order. In 1691, having discovered it independently, scientist Christiaan Huygens wrote about it also. Since the standard system of tuning at that time was quarter-comma meantone, in which the fifth is tuned to √5 , the appeal of this method was immediate, as the fifth of 31 EDO, at 696.77 cents, is only 0.19 cent wider than the fifth of quarter-comma meantone. Huygens not only realized this, he went farther and noted that 31 EDO provides an excellent approximation of septimal, or 7 limit harmony.

In the twentieth century, physicist, music theorist, and composer Adriaan Fokker, after reading Huygens's work, led a revival of interest in this system of tuning which led to a number of compositions, particularly by Dutch composers. Fokker designed the Fokker organ, a 31 tone equal-tempered organ, which was installed in Teyler's Museum in Haarlem in 1951 and moved to Muziekgebouw aan 't IJ in 2010 where it has been frequently used in concerts since it moved.

Interval size

Here are the sizes of some common intervals:

The 31 equal temperament has a very close fit to the 7:6, 8:7, and 7:5 ratios, which have no approximate fits in 12 equal temperament and only poor fits in 19 equal temperament. The composer Joel Mandelbaum (born 1932) used this tuning system specifically because of its good matches to the 7th and 11th partials in the harmonic series. The tuning has poor matches to both the 9:8 and 10:9 intervals (major and minor tone in just intonation); however, it has a good match for the average of the two. Practically it is very close to quarter-comma meantone.

This tuning can be considered a meantone temperament. It has the necessary property that a chain of its four fifths is equivalent to its major third (the syntonic comma 81:80 is tempered out), which also means that it contains a "meantone" that falls between the sizes of 10:9 and 9:8 as the combination of one of each of its chromatic and diatonic semitones.

Scale diagram

The following are the 31 notes in the scale:

The five "double flat" notes and five "double sharp" notes may be replaced by half sharps and half flats, similar to the quarter tone system:

Interval (cents)		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39		39
Note name	A		A		A♯		B♭		B		B		B		C		C		C		C♯		D♭		D		D		D		D♯		E♭		E		E		E		F		F		F		F♯		G♭		G		G		G		G♯		A♭		A		A
Note (cents)	0		39		77		116		155		194		232		271		310		348		387		426		465		503		542		581		619		658		697		735		774		813		852		890		929		968		1006		1045		1084		1123		1161		1200

Key signature	Scale							Number of sharps		Key signature	Scale							Number of flats
C major	C	D	E	F	G	A	B	0
G major	G	A	B	C	D	E	F♯	1
D major	D	E	F♯	G	A	B	C♯	2
A major	A	B	C♯	D	E	F♯	G♯	3
E major	E	F♯	G♯	A	B	C♯	D♯	4
B major	B	C♯	D♯	E	F♯	G♯	A♯	5
F♯ major	F♯	G♯	A♯	B	C♯	D♯	E♯	6
C♯ major	C♯	D♯	E♯	F♯	G♯	A♯	B♯	7
G♯ major	G♯	A♯	B♯	C♯	D♯	E♯	F	8
D♯ major	D♯	E♯	F	G♯	A♯	B♯	C	9
A♯ major	A♯	B♯	C	D♯	E♯	F	G	10		C major	C	D	E	F	G	A	B	21
E♯ major	E♯	F	G	A♯	B♯	C	D	11		G major	G	A	B	C	D	E	F	20
B♯ major	B♯	C	D	E♯	F	G	A	12		D major	D	E	F	G	A	B	C	19
F major	F	G	A	B♯	C	D	E	13		A major	A	B	C	D	E	F	G	18
C major	C	D	E	F	G	A	B	14		E major	E	F	G	A	B	C	D	17
G major	G	A	B	C	D	E	F	15		B major	B	C	D	E	F	G	A	16
D major	D	E	F	G	A	B	C	16		F major	F	G	A	B	C	D	E	15
A major	A	B	C	D	E	F	G	17		C major	C	D	E	F	G	A	B	14
E major	E	F	G	A	B	C	D	18		G major	G	A	B	C	D	E	F♭	13
B major	B	C	D	E	F	G	A	19		D major	D	E	F♭	G	A	B	C♭	12
F major	F	G	A	B	C	D	E	20		A major	A	B	C♭	D	E	F♭	G♭	11
C major	C	D	E	F	G	A	B	21		E major	E	F♭	G♭	A	B	C♭	D♭	10
										B major	B	C♭	D♭	E	F♭	G♭	A♭	9
										F♭ major	F♭	G♭	A♭	B	C♭	D♭	E♭	8
										C♭ major	C♭	D♭	E♭	F♭	G♭	A♭	B♭	7
										G♭ major	G♭	A♭	B♭	C♭	D♭	E♭	F	6
										D♭ major	D♭	E♭	F	G♭	A♭	B♭	C	5
										A♭ major	A♭	B♭	C	D♭	E♭	F	G	4
										E♭ major	E♭	F	G	A♭	B♭	C	D	3
										B♭ major	B♭	C	D	E♭	F	G	A	2
										F major	F	G	A	B♭	C	D	E	1
										C major	C	D	E	F	G	A	B	0

Chords of 31 equal temperament

Many chords of 31 EDO are discussed in the article on septimal meantone temperament. Chords not discussed there include the neutral thirds triad (Play), which might be written C–E–G, C–D–G or C–F–G, and the Orwell tetrad, which is C–E–F–B.

Usual chords like the major chord are rendered nicely in 31 EDO because the third and the fifth are very well approximated. Also, it is possible to play subminor chords (where the first third is subminor) and supermajor chords (where the first third is supermajor).

It is also possible to render nicely the harmonic seventh chord. For example on tonic C, with   C–E–G–A♯ . The seventh here is different from stacking a fifth and a minor third, which instead yields B♭ to make a dominant seventh. This difference cannot be made in 12 EDO.

Footnotes

1. The following composers mentioned in the title of Keislar (1991)'s journal article have Misplaced Pages articles:
   

   Easley Blackwood

   John Eaton

   Lou Harrison

   Ben Johnston

   Joel Mandelbaum

See also

• Archicembalo, alternate keyboard instrument with 36 keys per octave that was sometimes tuned as 31 EDO.

References

1. ^ Milne, A.; Sethares, W.A.; Plamondon, J. (Winter 2007). "Isomorphic controllers and dynamic tuning: Invariant fingerings across a tuning continuum". Computer Music Journal. 31 (4): 15–32 – via mitpressjournals.org.
2. Monzo, Joe (2005). "Equal temperament". Tonalsoft Encyclopedia of Microtonal Music Theory. Joe Monzo / Tonalsoft. Retrieved 28 February 2019.
3. ^ Keislar, Douglas (Winter 1991). "Six American composers on nonstandard tunnings: Easley Blackwood; John Eaton; Lou Harrison; Ben Johnston; Joel Mandelbaum; William Schottstaedt". Perspectives of New Music. 29 (1): 176–211. JSTOR 833076.

External links

• "main page". The Huygens-Fokker Foundation for Microtonal Music (huygens-fokker.org) (in Dutch and English).
• Fokker, Adriaan Daniël. "Equal temperament and the thirty-one-keyed organ". The Huygens-Fokker Foundation for Microtonal Music (huygens-fokker.org).
• Rapoport, Paul. "About 31 tone equal temperament". The Huygens-Fokker Foundation for Microtonal Music (huygens-fokker.org).
• Terpstra, Siemen. "Toward a theory of meantone (and 31 ET) harmony". The Huygens-Fokker Foundation for Microtonal Music (huygens-fokker.org).
• Barbieri, Patrizio (2008). Enharmonic instruments and music, 1470–1900. Latina: Il Levante Libreria Editrice. Archived from the original on 15 February 2009 – via patriziobarbieri.it.
• Khramov, M. (2009). Approximation to 7 limit just intonation in a scale of 31 EDO. FRSM-2009 International Symposium Frontiers of Research on Speech and Music. ABV IIITM Gwalior. pp. 73–82.
• "31 tone equal temperament". 31et.com (main page).

v t e Microtonal music
Composers	Richard Barrett Béla Bartók Easley Blackwood Jr. Heinz Bohlen Julián Carrillo Franklin Cox Mildred Couper John Eaton Brian Ferneyhough Michael Finnissy Bjørn Fongaard Alois Hába Christiaan Huygens Charles Ives Ben Johnston György Ligeti Stu Mackenzie Claus-Steffen Mahnkopf Joel Mandelbaum Joe Maneri Roger Redgate John Schneider Sevish Ezra Sims Nicola Vicentino Claude Vivier Elaine Walker Ivan Wyschnegradsky La Monte Young
Inventors	Glenn Branca Wendy Carlos Ivor Darreg Adriaan Fokker Lou Harrison Yuri Landman Harry Partch Tui St. George Tucker Nicola Vicentino
Tunings and scales	Non-octave- repeating scales Alpha scale Beta scale Gamma scale Delta scale Lambda scale (Bohlen–Pierce scale) Equal temperament 15 17 19 22 23 24 31 34 41 53 58 72 96 Just intonation Harry Partch's 43-tone scale Double diatonic
Concepts and techniques	Limit Otonality and Utonality Semitone Sonido 13 Xenharmonicity Tonality diamond
Groups and publications	Boston Microtonal Society Genesis of a Music Huygens-Fokker Foundation
Compositions	Beauty in the Beast quarter tone pieces just pieces Mother Sonata for Microtonal Piano Suite for Microtonal Piano Twelve Microtonal Etudes for Electronic Music Media Flying Microtonal Banana K.G. L.W.
Other topics	Enharmonic keyboard Generalized keyboard Modernism (music)

• Equal temperaments
• Microtonality