In music,53 equal temperament, called53 TET,53 EDO, or53 ET, is thetempered scale derived by dividing the octave into 53 equal steps (equal frequency ratios) (Play^ⓘ). Each step represents a frequency ratio of 2^1 ∕ 53 , or 22.6415 cents (Play^ⓘ), an interval sometimes called theHoldrian comma.

53 TET is a tuning ofequal temperament in which the tempered perfect fifth is 701.89 cents wide, as shown in Figure 1, and sequential pitches are separated by 22.642 cents.

The 53-TET tuning equates to the unison, ortempers out, the intervals⁠ 32 805 / 32 768⁠, known as theschisma, and⁠ 15 625 / 15 552⁠, known as thekleisma. These are both 5 limit intervals, involving only the primes 2, 3, and 5 in their factorization, and the fact that 53 TET tempers out both characterizes it completely as a 5 limit temperament: It is the onlyregular temperament tempering out both of these intervals, orcommas, a fact which seems to have first been recognized by Japanese music theoristShohé Tanaka. Because it tempers these out, 53 TET can be used for bothschismatic temperament, tempering out the schisma, andHanson temperament (also called kleismic), tempering out the kleisma.

The interval of⁠ 7 / 4⁠ is closest to the 43rd note (counting from 0) and 2^{43 ∕ 53} = 1.7548   is only 4.8 cents sharp from theharmonic 7th  =⁠ 7 / 4⁠ in 53 TET, and using it for7-limit harmony means that theseptimal kleisma, the interval⁠ 225 / 224⁠, is also tempered out.

Theoretical interest in this division goes back to antiquity.Jing Fang (78–37 BCE), a Chinese music theorist, observed that a series of 53 just fifths( [⁠ 3 / 2⁠]⁵³) is very nearly equal to 31 octaves (2³¹). He calculated this difference with six-digit accuracy to be⁠ 177 147 / 176 776⁠.^[2][3] Later the same observation was made by the mathematician and music theoristNicholas Mercator (c. 1620–1687), who calculated this value precisely as⁠ 3⁵³ / 2⁸⁴ ⁠ =⁠ 19 383 245 667 680 019 896 796 723 / 19 342 813 113 834 066 795 298 816⁠,^[verification needed] which is known asMercator's comma.^[4] Mercator's comma is of such small value to begin with( ≈ 3.615 cents), but 53 equal temperament flattens each fifth by only⁠1/ 53 ⁠ of that comma ( ≈0.0682 cent ≈⁠1/ 315 ⁠syntonic comma ≈⁠1/ 344 ⁠pythagorean comma). Thus, 53 tone equal temperament is for all practical purposes equivalent to an extendedPythagorean tuning.

After Mercator,William Holder published a treatise in 1694 which pointed out that 53 equal temperament also very closely approximates thejust major third (to within 1.4 cents), and consequently 53 equal temperament accommodates the intervals of5 limitjust intonation very well.^[5][6] This property of 53 TET may have been known earlier;Isaac Newton's unpublished manuscripts suggest that he had been aware of it as early as 1664–1665.^[7]

In the 19th century, people began devising instruments in 53 TET, with an eye to their use in playing near-just5-limit music. Such instruments were devised byR.H.M. Bosanquet^{[8](p 328–329)} and the American tunerJ.P. White.^[8](p 329) Subsequently, the temperament has seen occasional use by composers in the west, and by the early 20th century, 53 TET had become the most common form of tuning inOttoman classical music, replacing its older, unequal tuning.Arabic music, which for the most part bases its theory onquartertones, has also made some use of it; the Syrian violinist and music theoristTwfiq Al-Sabagh proposed that instead of an equal division of the octave into 24 parts a 24 note scale in 53 TET should be used as the master scale for Arabic music.^{[citation needed]}

Croatian composerJosip Štolcer-Slavenski wrote one piece, which has never been published, which usesBosanquet's Enharmonium during its first movement, entitledMusic for Natur-ton-system.^{[9][10][11][a]}

Furthermore, General Thompson worked in league with the London-based guitar makerLouis Panormo to produce the Enharmonic Guitar.^[12]

Attempting to use standard notation, seven-letter notes plus sharps or flats, can quickly become confusing. This is unlike the case with19 TET and31 TET where there is little ambiguity. By not being meantone, it adds some problems that require more attention. Specifically, the Pythagorean major third (ditone) and just major third are distinguished, as are the Pythagorean minor third (semiditone) and just minor third. The fact that thesyntonic comma is not tempered out means that notes and intervals need to be defined more precisely.Ottoman classical music uses a notation of flats and sharps for the 9 comma tone.

Furthermore, since 53 is not a multiple of 12, notes such as G♯ and A♭ are not enharmonically equivalent, nor are the correspondingkey signatures. As a result, many key signatures will require the use of double sharps (such as G♯ major / E♯ minor), double flats (such as F♭ major / D♭ minor), or microtonal alterations.

Extended pythagorean notation, using only sharps and flats, gives the following chromatic scale:

• C, B♯, A♯, E, D♭, C♯, B, F, E,
• D, C, B♯, F, E♭, D♯, C♯, G, F♭,
• E, D, C/A, G,
• F, E♯, D♯, A, G♭, F♯, E, D/B, A,
• G, F, E♯, B, A♭, G♯, F♯, C, B,
• A, G, F/D, C, B♭, A♯, G♯, D, C♭,
• B, A, G/E, D, C

Unfortunately, the notes run out of letter-order, and up to quadruple sharps and flats are required. As a result, a just major 3rd must be spelled as a diminished 4th.

Ups and downs notation^[13] keeps the notes in order and also preserves the traditional meaning of sharp and flat. It uses up and down arrows, written as a caret or a lower-case "v", usually in a sans-serif font. One arrow equals one step of 53-TET. In note names, the arrows come first, to facilitate chord naming. The many enharmonic equivalences allow great freedom of spelling.

• C, ^C, ^^C, vvC♯/vD♭, vC♯/D♭, C♯/^D♭, ^C♯/^^D♭, vvD, vD,
• D, ^D, ^^D, vvD♯/vE♭, vD♯/E♭, D♯/^E♭, ^D♯/^^E♭, vvE, vE,
• E, ^E, ^^E/vvF, vF,
• F, ^F, ^^F, vvF♯/vG♭, vF♯/G♭, F♯/^G♭, ^F♯/^^G♭, vvG, vG,
• G, ^G, ^^G, vvG♯/vA♭, vG♯/A♭, G♯/^A♭, ^G♯/^^A♭, vvA, vA,
• A, ^A, ^^A, vvA♯/vB♭, vA♯/B♭, A♯/^B♭, ^A♯/^^B♭, vvB, vB,
• B, ^B, ^^B/vvC, vC, C

Since 53-TET is a Pythagorean system, with nearly pure fifths, justly-intonated major and minor triads cannot be spelled in the same manner as in ameantone tuning. Instead, the major triads are chords like C-F♭-G (using the Pythagorean-based notation), where the major third is a diminished fourth; this is the defining characteristic ofschismatic temperament. Likewise, the minor triads are chords like C-D♯-G. In 53-TET, thedominant seventh chord would be spelled C-F♭-G-B♭, but theotonal tetrad is C-F♭-G-C, and C-F♭-G-A♯ is still another seventh chord. Theutonal tetrad, the inversion of the otonal tetrad, is spelled C-D♯-G-G.

Further septimal chords are the diminished triad, having the two forms C-D♯-G♭ and C-F-G♭, the subminor triad, C-F-G, the supermajor triad C-D-G, and corresponding tetrads C-F-G-B and C-D-G-A♯. Since 53-TET tempers out theseptimal kleisma, the septimal kleisma augmented triad C-F♭-B in its various inversions is also a chord of the system. So is the Orwell tetrad, C-F♭-D-G in its various inversions.

Ups and downs notation permits more conventional spellings. Since it also names the intervals of 53 TET,^[14] it provides precise chord names too. The pythagorean minor chord with a⁠ 32 / 27⁠ third is still named Cm and still spelled C–E♭–G. But the 5-limitupminor chord uses the upminor 3rd 6/5 and is spelled C–^E♭–G. This chord is named C^m. Compare with ^Cm (^C–^E♭–^G).

• Major triad: C-vE-G (downmajor)
• Minor triad: C-^E♭-G (upminor)
• Dominant 7th: C-vE-G-B♭ (down add-7)
• Otonal tetrad: C-vE-G-vB♭ (down7)
• Utonal tetrad: C-^E♭-G-^A (upminor6)
• Diminished triad: C-^E♭-G♭ (updim)
• Diminished triad: C-vE♭-G♭ (downdim)
• Subminor triad: C-vE♭-G (downminor)
• Supermajor triad: C-^E-G (upmajor)
• Subminor tetrad: C-vE♭-G-vA (downminor6)
• Supermajor tetrad: C-^E-G-^B♭ (up7)
• Augmented triad: C-vE-vvG♯ (downaug dud-5)
• Orwell triad: C-vE-vvG-^A (downmajor dud-5 up6)

Because a distance of 31 steps in this scale is almost precisely equal to ajustperfect fifth, in theory this scale can be considered a slightly tempered form ofPythagorean tuning that has been extended to 53 tones. As such the intervals available can have the same properties as any Pythagorean tuning, such as fifths that are (practically) pure, major thirds that are wide from just (about⁠ 81 / 64⁠ opposed to the purer⁠ 5 / 4⁠, and minor thirds that are conversely narrow (⁠ 32 / 27⁠ compared to⁠ 6 / 5⁠).

However, 53 TET contains additional intervals that are very close to just intonation. For instance, the interval of 17 steps is also a major third, but only 1.4 cents narrower than the very pure just interval⁠ 5 / 4⁠. 53 TET is very good as an approximation to any interval in 5 limit just intonation. Similarly, the pure just interval⁠ 6 / 5⁠ is only 1.3 cents wider than 14 steps in 53 TET.

The matches to the just intervals involving the 7th harmonic are slightly less close (43 steps are 4.8 cents sharp for⁠ 7 / 4⁠), but all such intervals are still quite closely matched with the highest deviation being the⁠ 7 / 5⁠ tritone. The 11th harmonic and intervals involving it are less closely matched, as illustrated by the undecimal neutral seconds and thirds in the table below. 7-limit ratios are colored light gray, and 11- and 13-limit ratios are colored dark gray.

The following are 21 of the 53 notes in the chromatic scale. The rest can easily be added.

Interval (steps)		3		2		4		3		2		3		2		1		2		4		1		4		3		2		4		3		2		3		2		1		2
Interval (cents)		68		45		91		68		45		68		45		23		45		91		23		91		68		45		91		68		45		68		45		23		45
Note name (Pythagorean notation)	C		E		C♯		D		F		D♯		F♭		D		C/A		F		G♭		F♯		G		B		G♯		B		C		A♯		C♭		A		G/E		C
Note name (ups and downs notation)	C		vvC♯/vD♭		C♯/^D♭		D		vvD♯/vE♭		D♯/^E♭		vE		^E		^^E/vvF		F		vF♯/G♭		F♯/^G♭		G		vvG♯/vA♭		G♯/^A♭		vA		vvA♯/vB♭		A♯/^B♭		vB		^B		^^B/vvC		C
Note (cents)	0		68		113		204		272		317		385		430		453		498		589		611		702		770		815		883		974		1018		1087		1132		1155		1200
Note (steps)	0		3		5		9		12		14		17		19		20		22		26		27		31		34		36		39		43		45		48		50		51		53

Inmusic theory andmusical tuning theHoldrian comma, also calledHolder's comma, and rarely theArabian comma,^[15] is a smallmusical interval of approximately 22.6415 cents,^[15] equal to one step of 53 equal temperament, or${\displaystyle \sqrt[53]{2}}$ (play^ⓘ). The name"comma", however, is technically misleading, since this interval is an irrational number and it does not describe a compromise between intervals of any tuning system. The interval gets the name "comma" because it is a close approximation of severalcommas, most notably thesyntonic comma (21.51 cents) (play^ⓘ), which was widely used as a unit of tonal measurement duringHolder's time.

The origin of Holder's comma resides in the fact that theAncient Greeks (or at least to the RomanBoethius^[b])believed that in thePythagorean tuning the tone could be divided in nine commas, four of which forming the diatonic semitone and five the chromatic semitone. If all these commas are exactly of the same size, there results an octave of5 tones + 2 diatonic semitones,  5 × 9 + 2 × 4 = 53 equal commas. Holder^[18] attributes the division of the octave in 53 equal parts toNicholas Mercator,^[c]who himself had proposed that⁠1/ 53 ⁠ part of the octave be named the "artificial comma".

Mercator's comma is a name often used for a closely related interval because of its association withNicholas Mercator.^[d]One of these intervals was first described byJing Fang in45BCE.^[15] Mercator applied logarithms to determine that${\displaystyle \sqrt[55]{2}}$ (≈ 21.8182 cents) was nearly equivalent to a syntonic comma of ≈ 21.5063 cents (a feature of the prevalentmeantone temperament of the time). He also considered that an "artificial comma" of${\displaystyle \sqrt[53]{2}}$ might be useful, because 31 octaves could be practically approximated by a cycle of 53 just fifths.Holder, for whom theHoldrian comma is named, favored this latter unit because the intervals of 53 equal temperament are closer tojust intonation than to55 TET. Thus Mercator's comma and the Holdrian comma are two distinct but nearly equal intervals.

The Holdrian comma has been employed mainly in Ottoman/Turkish music theory byKemal Ilerici, and by the Turkish composerErol Sayan. The name of this comma isHolder koması in Turkish.

For instance, theRast makam (similar to the Westernmajor scale, or more precisely to thejustly-tuned major scale) may be considered in terms of Holdrian commas:

where denotes a Holdrian comma flat,^[e]while in contrast, the Nihavend makam (similar to the Westernminor scale):

where♭ denotes a five-comma flat,hasmedium seconds betweend–e♭,e–f,g–a♭,a♭–b♭, andb♭–c′, a medium second being somewhere in between 8 and 9 commas.^[15]

• Rodgers, Prent (May 2007)."Whisper song in 53 EDO".Bumper Music (podcast) (slower ed.).
• Hanson, Larry (1989)."Development of a 53 tone keyboard layout"(PDF).Xenharmonicon.XII. Hanover, NH: Frog Peak Music:68–85. Retrieved4 January 2021 – via Anaphoria.com.
• "Algebra of Tonal Functions".Sonantometry (blog). 1 May 2007. — Tonal functions as 53 TET grades.
• Barbieri, Patrizio (2008)."Enharmonic instruments and music, 1470–1900".Latina, Il Levante Libreria Editrice. Italy. Archived fromthe original on 2009-02-15.
• Kukula, Jim (August 2005)."Equal temperament with 53 pitches per octave".Interdependent Science. Fractal microtonal music. Retrieved4 January 2021.

• Equal temperaments
• Microtonality
• Commas (music)

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