Pergen names: Difference between revisions

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A **pergen** (pronounced "peer-gen") is a way of identifying a rank-2 or rank-3 regular temperament solely by its period and generator(s). For any temperament, there are many possible periods and generators. For the pergen, they are chosen to use the fewest, and smallest, prime factors possible. Fractions are allowed, e.g. half-octave, but avoided if possible.

If a rank-2 temperament uses the primes 2 and 3 in its comma(s), or in its prime subgroup (i.e. isn't explicitly non-octave or non-fifth), then the period can be expressed as the octave 2/1, or some fraction of an octave. Furthermore, the generator can usually be expressed as some 3-limit interval, or some fraction of such an interval. The fraction is always of the form 1/N, thus the octave and/or the 3-limit interval is **split** into N parts. An interval which is split into multiple generators is called a **multi-gen**.

For example, the srutal temperament splits the octave in two, and its pergen name is half-octave. The pergen is written {P8/2, P5}. Not only the srutal temperament, but also the srutal comma is said to split the octave. The dicot temperament splits the fifth in two, and is called half-fifth, written {P8, P5/2}. Porcupine is third-fourth, {P8, P4/3}. Semaphore, which means "semi-fourth", is of course half-fourth.

Many temperaments will share the same pergen. This has the advantage of reducing the thousands of temperament names to perhaps a few dozen categories. It focuses on the melodic properties of the temperament, not the harmonic properties. MOS scales in both srutal and injera sound the same, although they temper out different commas. In addition, the pergen tells us how to notate the temperament using [[Ups and Downs Notation|ups and downs]].

The largest category contains all single-comma temperaments with a comma of the form 2&lt;span style="vertical-align: super;"&gt;x &lt;/span&gt;3&lt;span style="vertical-align: super;"&gt;y &lt;/span&gt;P or 2&lt;span style="vertical-align: super;"&gt;x &lt;/span&gt;3&lt;span style="vertical-align: super;"&gt;y &lt;/span&gt;P&lt;span style="vertical-align: super;"&gt;-1&lt;/span&gt;, where P is a prime &gt; 3 (a **higher prime**), e.g. 81/80 or 64/63. The period is the octave, and the generator is the fifth: {P8, P5}. Such temperaments are called **unsplit**.

For example, srutal could be {P8/2, M2/2}, but P5 is preferred because it is unsplit. Or it could be {P8/2, P12}, but P5 is preferred because it is smaller. Or it could be {P8/2, P4}, but P5 is always preferred over P4. Note that P5/2 is __not__ preferred over P4/2. For example, decimal is {P8/2, P4/2}, not {P8/2, P5/2}.

||= {P8, P5} ||= unsplit ||= 81/80 ||= meantone ||= green ||= gT ||

||= {P8/2, P5} ||= half-octave ||= (11, -4, -2) ||= srutal ||= small deep green ||= sggT ||

||= {P8, P5/2} ||= half-fifth ||= 25/24 ||= dicot ||= deep yellow ||= yyT ||

||= {P8, P4/2} ||= half-fourth ||= 49/48 ||= semaphore ||= deep blue ||= bbT ||

||= {P8/2, P4/2} ||= half-octave, half-fourth ||= 25/24, 49/48 ||= decimal ||= deep yellow and deep blue ||= yy&amp;bbT ||

||= {P8, P4/3} ||= third-fourth ||= 250/243 ||= porcupine ||= triple yellow ||= y&lt;span style="vertical-align: super;"&gt;3&lt;/span&gt;T ||

||= {P8, P11/3} ||= third-eleventh ||= (12,-1,0,0,-3) ||= small triple amber ||= small triple amber ||= sa&lt;span style="vertical-align: super;"&gt;3&lt;/span&gt;T ||

||= {P8/4, P5} ||= quarter-octave ||= (3,4,-4) ||= diminished ||= quadruple green ||= g&lt;span style="vertical-align: super;"&gt;4&lt;/span&gt;T ||

||= {P8/2, M2/4} ||= half-octave, quarter-tone ||= (-17,2,0,0,4) ||= large quadruple jade ||= large quadruple jade ||= Lj&lt;span style="vertical-align: super;"&gt;4&lt;/span&gt;T ||

||= {P8, P12/5} ||= fifth-twelfth ||= (-10,-1,5) ||= magic ||= large quintuple yellow ||= Ly&lt;span style="vertical-align: super;"&gt;5&lt;/span&gt;T ||

The color name indicates the amount of splitting: deep splits something into two parts, triple into three parts, etc. The multi-gen is usually some voicing of the 4th or 5th, but can be any 3-limit interval, as in the second to last example.

[//For nine-fold colors, the multi-gen may be the minor 3rd 32/27. These intervals may also be voiced wider, as 3/1, 8/3, etc. To avoid cumbersome degree names like 16th or 18th, for degrees above 12, the widening is indicated with one "W" per octave. Thus 32/9 = Wm7, 9/2 = WWM2, etc.]//

For non-standard prime groups, the period uses the first prime only, and the multi-gen usually (see the 1st example in the Derivation section) uses the first two primes only. [[Kite's color notation|Color notation ]]is used to indicate higher primes. 5/4 = y3 = yellow 3rd, 7/4 = b7 = blue 7th, and 11/8 = j4 = jade 4th. 2.5.7 with 50/49 tempered out is {P8/2, y3} = half-octave, yellow-third.

Rank-3 pergens have three intervals, period, gen1 and gen2, any or all of which may be split. The unsplit 2.3.5... subgroup's pergen could be {P8, P5, y3}. However, to avoid higher primes, and more closely mimic conventional notation, it's better to reduce gen2 to g1 = 81/80. Since 81/80 is a perfect unison, it can be notated with an up symbol, and we have {P8, P5, ^1} = unsplit with ups. However, if either the period or gen1 is split, ups and downs will be needed to notate the fractional intervals, and colors must be used for gen2. (See also highs and lows below.)

Examples: Marvel (2.3.5.7 and 225/224, or reddish yellow) is {P8, P5, ^1} = unsplit with ups (^1 = 81/80). Deep reddish (2.3.5.7 and 50/49) is {P8/2, P5, g1} = half-octave with green. Triple bluish (2.3.5.7 and 1029/1000) is {P8, P11/3, ^1} = third-eleventh with ups.

A rank-4 temperament has a pergen of four intervals, rank-5 has five intervals, etc. A rank-1 temperament could have a pergen of one, such as {P8/12} for 12-edo or {P12/13} for 13-ed3, but there's no particular reason to do so.

The preferred pergen for untempered just intonation is the octave, the fifth, and a list of commas, each containing only one higher prime: {P8, P5, 81/80, 64/63, ...}. The higher prime's exponent in the monzo must be 1 or -1. These commas are called **notational commas**. They are not necessarily tempered out, they merely determine how a higher prime is mapped to a 3-limit interval, and thus how ratios containing higher primes are notated on the staff. By definition, the only commas that map to P1 are notational ones, and those that are the sum or difference of notational ones. There is universal agreement that 5's notational comma is 81/80. But the choice of notational commas for other primes, especially 11 and 13, is somewhat arbitrary. For example, if 11's notational comma is 33/32, 11/8 is notated as a perfect 4th. But if it's 729/704, 11/8 is an augmented 4th. Note that the choice of 11's notational comma affects the mapping of other 11-limit commas.

For example, 2.3.5.7 with commas 256/243 and 225/224. The 1st comma splits the octave into 5 parts, and makes the 5th be exactly 3/5 of the octave. The multi-gen must use primes 2 and 5. In this case, the pergen is {P8/5, y3}, the same as Blackwood.

To find a temperament's pergen, first find the period-generator mapping. This is a matrix with a column for each prime in the subgroup, and a row for each period/generator. Not all such mappings will work, the matrix must be in row echelon form. Graham Breed's website has a temperament finder [[@http://x31eq.com/temper/uv.html|x31eq.com/temper/uv.html]] that will find such a matrix, it's the reduced mapping. Next make a square matrix by discarding columns, usually the columns for the highest primes. But lower primes may need to be discarded, as in the previous example, to ensure that the diagonal has no zeros. Lower primes &gt; 3 may also be discarded to minimize splitting, see the Breedsmic example below. Then invert the matrix to get the monzos for each period/generator. Add/subtract periods from the generator to get alternate generators. If the interval becomes descending, invert it. For rank-3, add/subtract both periods and generators from the 2nd generator to get more alternates. Choose the alternates to minimize the splitting and the cents.

Again, period = P8 and gen1 = P5/2. Gen2 = (-3, -1, 2)/2. To add gen1 to gen2, add a double gen1 to the 2nd multi-gen, the multi-gen2. A double half-fifth is a fifth = (-1, 1, 0), and this gives us (-4, 0, 2)/2 = 7/4. The fraction disappears, the multi-gen becomes the gen, and we can add/subtract the period and the gen1 directly. Subtracting an octave and inverting makes gen2 = 8/7 = r2. Adding an octave and subtracting 4 half-fifths makes 64/63 = r1. The pergen is {P8, P5/2, r1} = half-fifth with red. This is far better than {P8, P5/2, gg7/4}. The pergen sometimes uses a larger prime in place of a smaller one in order to avoid splitting gen2, but only if the smaller prime is &gt; 3. In other words, the first priority is to have as few higher primes (colors) as possible, next to have as few fractions as possible, finally to have the higher primes be as small as possible.

Also, pergens allow a systematic exploration of notations for rank-2, rank-3, etc. regular temperaments, without having to examine each of the thousands of individual temperaments. For example, all unsplit temperaments are notated identically. They require only conventional notation: 7 nominals, plus sharps and flats.

Most rank-2 temperaments require an additional pair of accidentals, [[Ups and Downs Notation|ups and downs]]. And certain rank-2 temperaments require another additional pair. One possibility is highs and lows, written / and \. v\D is down-low D, and /P5 is high-five. Alternatively, color accidentals (y/g, r/b, j/a, etc.) could be used. However, this constrains a pergen to a specific temperament. For example, both mohajira and dicot are {P8, P5/2}. Using y/g implies dicot, using j/a implies mohajira, but using ^/v implies neither, and is a more general notation.

Analogous to 22-edo, sometimes additional accidentals aren't needed, but are desirable, to avoid misspelled chords. For example, schismic is unsplit and can be notated conventionally. But because the schisma does not map to a unison, this causes 4:5:6 to be spelled as C Fb G. With ^1 = 81/80, the chord can be spelled properly as C Ev G. The 135/128 comma maps to a unison, although not a perfect one, and as a result doesn't need ups and downs.

Not all combinations of periods and generators are valid. Some are duplicates of other pergens. {P8/2, M2/2} is actually {P8/2, P5}. Some combinations are impossible. There is no {P8, M2/2}, because no combination of periods and generators equals P5. The following table lists all the rank-2 pergens that contain primes 2 and 3, grouped by the size of the larger splitting fraction.

The enharmonic interval can be added to or subtracted from any note or interval, renaming it, but not changing the pitch of the note (or width of the interval). It's analogous to the dim 2nd in 12-edo, which equates C# with Db, A4 with d5, etc. In a single-comma temperament, the comma maps to the enharmonic interval. This interval is very important. Everything about the notation can be deduced from the pergen and the enharmonic interval.

An edo is incompatible with a pergen if the split is impossible. For example, all odd-numbered edos are incompatible with half-octave pergens. An edo is somewhat incompatible with a pergen if the period and generator can only generate a subset of the edo. For example, 15-edo is somewhat incompatible with {P8, P5}, because any chain-of-5ths scale could only make a 5-edo subset. Such edos are marked with asterisks. 13b is incompatible with {P8, P5/2}, but 13 isn't. However, 13 is incompatible with heptatonic notation.

 

[//This part needs clarification. 5ths wider than 720¢ can be played, but they can't be notated as perfect 5ths.//]

||= {P8, P5} ||= none ||= none ||= none ||= C - G ||= meantone ||= 12, 13b, 14*, 15*, 16,

||= {P8/2, P5} ||= ^^d2 (if 5th

||= {P8, P4/2} ||= vvm2 ||= C^^ = Db ||= P4/2 = ^M2 = vm3 ||= C - D^=Ebv - F ||= semaphore

||= {P8, P5/2} ||= vvA1 ||= C^^ = C# ||= P5/2 = ^m3 = vM3 ||= C - Eb^=Ev - G ||= mohajira

||= {P8/2, P4/2} ||= \\m2,

Removing the ups and downs from an enharmonic interval makes a "bare" enharmonic, a conventional interval which vanishes in certain edos. For example, {P8/2, P5}'s enharmonic interval is ^^d2, the bare enharmonic is d2, and d2 vanishes in 12-edo. Every rank-2 temperament has a "sweet spot" for tuning the 5th, usually a narrow range of about 5-10¢. 12-edo's fifth is the "tipping point": if the temperament's 5th is flatter than 12-edo's, d2 is ascending, and if it's sharper, it's descending. The ups and downs are meant to indicate that the enharmonic interval vanishes. Thus if d2 is ascending, it should be downed, and if it's descending, upped. Therefore __**ups and downs may need to be swapped, depending on the size of the 5th**__ in the particular rank-2 tuning you are using. In the above table, this is shown explicitly for {P8/2, P5}, and implied for all the other pergens. In the table, the other pergens' enharmonic intervals are upped or downed as if the 5th were just.

 

A &lt;strong&gt;pergen&lt;/strong&gt; (pronounced &amp;quot;peer-gen&amp;quot;) is a way of identifying a rank-2 or rank-3 regular temperament solely by its period and generator(s). For any temperament, there are many possible periods and generators. For the pergen, they are chosen to use the fewest, and smallest, prime factors possible. Fractions are allowed, e.g. half-octave, but avoided if possible.&lt;br /&gt;

If a rank-2 temperament uses the primes 2 and 3 in its comma(s), or in its prime subgroup (i.e. isn't explicitly non-octave or non-fifth), then the period can be expressed as the octave 2/1, or some fraction of an octave. Furthermore, the generator can usually be expressed as some 3-limit interval, or some fraction of such an interval. The fraction is always of the form 1/N, thus the octave and/or the 3-limit interval is &lt;strong&gt;split&lt;/strong&gt; into N parts. An interval which is split into multiple generators is called a &lt;strong&gt;multi-gen&lt;/strong&gt;.&lt;br /&gt;

For example, the srutal temperament splits the octave in two, and its pergen name is half-octave. The pergen is written {P8/2, P5}. Not only the srutal temperament, but also the srutal comma is said to split the octave. The dicot temperament splits the fifth in two, and is called half-fifth, written {P8, P5/2}. Porcupine is third-fourth, {P8, P4/3}. Semaphore, which means &amp;quot;semi-fourth&amp;quot;, is of course half-fourth.&lt;br /&gt;

Many temperaments will share the same pergen. This has the advantage of reducing the thousands of temperament names to perhaps a few dozen categories. It focuses on the melodic properties of the temperament, not the harmonic properties. MOS scales in both srutal and injera sound the same, although they temper out different commas. In addition, the pergen tells us how to notate the temperament using &lt;a class="wiki_link" href="/Ups%20and%20Downs%20Notation"&gt;ups and downs&lt;/a&gt;.&lt;br /&gt;

The largest category contains all single-comma temperaments with a comma of the form 2&lt;span style="vertical-align: super;"&gt;x &lt;/span&gt;3&lt;span style="vertical-align: super;"&gt;y &lt;/span&gt;P or 2&lt;span style="vertical-align: super;"&gt;x &lt;/span&gt;3&lt;span style="vertical-align: super;"&gt;y &lt;/span&gt;P&lt;span style="vertical-align: super;"&gt;-1&lt;/span&gt;, where P is a prime &amp;gt; 3 (a &lt;strong&gt;higher prime&lt;/strong&gt;), e.g. 81/80 or 64/63. The period is the octave, and the generator is the fifth: {P8, P5}. Such temperaments are called &lt;strong&gt;unsplit&lt;/strong&gt;.&lt;br /&gt;

For example, srutal could be {P8/2, M2/2}, but P5 is preferred because it is unsplit. Or it could be {P8/2, P12}, but P5 is preferred because it is smaller. Or it could be {P8/2, P4}, but P5 is always preferred over P4. Note that P5/2 is &lt;u&gt;not&lt;/u&gt; preferred over P4/2. For example, decimal is {P8/2, P4/2}, not {P8/2, P5/2}.&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;{P8, P5}&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;{P8/2, P5}&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;{P8, P5/2}&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;{P8, P4/2}&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;{P8/2, P4/2}&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;half-octave, half-fourth&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;{P8, P4/3}&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;{P8, P11/3}&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;{P8/4, P5}&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;{P8/2, M2/4}&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;{P8, P12/5}&lt;br /&gt;

The color name indicates the amount of splitting: deep splits something into two parts, triple into three parts, etc. The multi-gen is usually some voicing of the 4th or 5th, but can be any 3-limit interval, as in the second to last example.&lt;br /&gt;

[&lt;em&gt;For nine-fold colors, the multi-gen may be the minor 3rd 32/27. These intervals may also be voiced wider, as 3/1, 8/3, etc. To avoid cumbersome degree names like 16th or 18th, for degrees above 12, the widening is indicated with one &amp;quot;W&amp;quot; per octave. Thus 32/9 = Wm7, 9/2 = WWM2, etc.]&lt;/em&gt;&lt;br /&gt;

For non-standard prime groups, the period uses the first prime only, and the multi-gen usually (see the 1st example in the Derivation section) uses the first two primes only. &lt;a class="wiki_link" href="/Kite%27s%20color%20notation"&gt;Color notation &lt;/a&gt;is used to indicate higher primes. 5/4 = y3 = yellow 3rd, 7/4 = b7 = blue 7th, and 11/8 = j4 = jade 4th. 2.5.7 with 50/49 tempered out is {P8/2, y3} = half-octave, yellow-third.&lt;br /&gt;

Rank-3 pergens have three intervals, period, gen1 and gen2, any or all of which may be split. The unsplit 2.3.5... subgroup's pergen could be {P8, P5, y3}. However, to avoid higher primes, and more closely mimic conventional notation, it's better to reduce gen2 to g1 = 81/80. Since 81/80 is a perfect unison, it can be notated with an up symbol, and we have {P8, P5, ^1} = unsplit with ups. However, if either the period or gen1 is split, ups and downs will be needed to notate the fractional intervals, and colors must be used for gen2. (See also highs and lows below.)&lt;br /&gt;

Examples: Marvel (2.3.5.7 and 225/224, or reddish yellow) is {P8, P5, ^1} = unsplit with ups (^1 = 81/80). Deep reddish (2.3.5.7 and 50/49) is {P8/2, P5, g1} = half-octave with green. Triple bluish (2.3.5.7 and 1029/1000) is {P8, P11/3, ^1} = third-eleventh with ups.&lt;br /&gt;

A rank-4 temperament has a pergen of four intervals, rank-5 has five intervals, etc. A rank-1 temperament could have a pergen of one, such as {P8/12} for 12-edo or {P12/13} for 13-ed3, but there's no particular reason to do so.&lt;br /&gt;

The preferred pergen for untempered just intonation is the octave, the fifth, and a list of commas, each containing only one higher prime: {P8, P5, 81/80, 64/63, ...}. The higher prime's exponent in the monzo must be 1 or -1. These commas are called &lt;strong&gt;notational commas&lt;/strong&gt;. They are not necessarily tempered out, they merely determine how a higher prime is mapped to a 3-limit interval, and thus how ratios containing higher primes are notated on the staff. By definition, the only commas that map to P1 are notational ones, and those that are the sum or difference of notational ones. There is universal agreement that 5's notational comma is 81/80. But the choice of notational commas for other primes, especially 11 and 13, is somewhat arbitrary. For example, if 11's notational comma is 33/32, 11/8 is notated as a perfect 4th. But if it's 729/704, 11/8 is an augmented 4th. Note that the choice of 11's notational comma affects the mapping of other 11-limit commas.&lt;br /&gt;

For example, 2.3.5.7 with commas 256/243 and 225/224. The 1st comma splits the octave into 5 parts, and makes the 5th be exactly 3/5 of the octave. The multi-gen must use primes 2 and 5. In this case, the pergen is {P8/5, y3}, the same as Blackwood.&lt;br /&gt;

To find a temperament's pergen, first find the period-generator mapping. This is a matrix with a column for each prime in the subgroup, and a row for each period/generator. Not all such mappings will work, the matrix must be in row echelon form. Graham Breed's website has a temperament finder &lt;a class="wiki_link_ext" href="http://x31eq.com/temper/uv.html" rel="nofollow" target="_blank"&gt;x31eq.com/temper/uv.html&lt;/a&gt; that will find such a matrix, it's the reduced mapping. Next make a square matrix by discarding columns, usually the columns for the highest primes. But lower primes may need to be discarded, as in the previous example, to ensure that the diagonal has no zeros. Lower primes &amp;gt; 3 may also be discarded to minimize splitting, see the Breedsmic example below. Then invert the matrix to get the monzos for each period/generator. Add/subtract periods from the generator to get alternate generators. If the interval becomes descending, invert it. For rank-3, add/subtract both periods and generators from the 2nd generator to get more alternates. Choose the alternates to minimize the splitting and the cents.&lt;br /&gt;

Again, period = P8 and gen1 = P5/2. Gen2 = (-3, -1, 2)/2. To add gen1 to gen2, add a double gen1 to the 2nd multi-gen, the multi-gen2. A double half-fifth is a fifth = (-1, 1, 0), and this gives us (-4, 0, 2)/2 = 7/4. The fraction disappears, the multi-gen becomes the gen, and we can add/subtract the period and the gen1 directly. Subtracting an octave and inverting makes gen2 = 8/7 = r2. Adding an octave and subtracting 4 half-fifths makes 64/63 = r1. The pergen is {P8, P5/2, r1} = half-fifth with red. This is far better than {P8, P5/2, gg7/4}. The pergen sometimes uses a larger prime in place of a smaller one in order to avoid splitting gen2, but only if the smaller prime is &amp;gt; 3. In other words, the first priority is to have as few higher primes (colors) as possible, next to have as few fractions as possible, finally to have the higher primes be as small as possible.&lt;br /&gt;

Also, pergens allow a systematic exploration of notations for rank-2, rank-3, etc. regular temperaments, without having to examine each of the thousands of individual temperaments. For example, all unsplit temperaments are notated identically. They require only conventional notation: 7 nominals, plus sharps and flats.&lt;br /&gt;

Most rank-2 temperaments require an additional pair of accidentals, &lt;a class="wiki_link" href="/Ups%20and%20Downs%20Notation"&gt;ups and downs&lt;/a&gt;. And certain rank-2 temperaments require another additional pair. One possibility is highs and lows, written / and \. v\D is down-low D, and /P5 is high-five. Alternatively, color accidentals (y/g, r/b, j/a, etc.) could be used. However, this constrains a pergen to a specific temperament. For example, both mohajira and dicot are {P8, P5/2}. Using y/g implies dicot, using j/a implies mohajira, but using ^/v implies neither, and is a more general notation.&lt;br /&gt;

Analogous to 22-edo, sometimes additional accidentals aren't needed, but are desirable, to avoid misspelled chords. For example, schismic is unsplit and can be notated conventionally. But because the schisma does not map to a unison, this causes 4:5:6 to be spelled as C Fb G. With ^1 = 81/80, the chord can be spelled properly as C Ev G. The 135/128 comma maps to a unison, although not a perfect one, and as a result doesn't need ups and downs.&lt;br /&gt;

Not all combinations of periods and generators are valid. Some are duplicates of other pergens. {P8/2, M2/2} is actually {P8/2, P5}. Some combinations are impossible. There is no {P8, M2/2}, because no combination of periods and generators equals P5. The following table lists all the rank-2 pergens that contain primes 2 and 3, grouped by the size of the larger splitting fraction.&lt;br /&gt;

The enharmonic interval can be added to or subtracted from any note or interval, renaming it, but not changing the pitch of the note (or width of the interval). It's analogous to the dim 2nd in 12-edo, which equates C# with Db, A4 with d5, etc. In a single-comma temperament, the comma maps to the enharmonic interval. This interval is very important. Everything about the notation can be deduced from the pergen and the enharmonic interval.&lt;br /&gt;

An edo is incompatible with a pergen if the split is impossible. For example, all odd-numbered edos are incompatible with half-octave pergens. An edo is somewhat incompatible with a pergen if the period and generator can only generate a subset of the edo. For example, 15-edo is somewhat incompatible with {P8, P5}, because any chain-of-5ths scale could only make a 5-edo subset. Such edos are marked with asterisks. 13b is incompatible with {P8, P5/2}, but 13 isn't. However, 13 is incompatible with heptatonic notation.&lt;br /&gt;

[&lt;em&gt;This part needs clarification. 5ths wider than 720¢ can be played, but they can't be notated as perfect 5ths.&lt;/em&gt;]&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;{P8, P5}&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;{P8/2, P5}&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;{P8, P4/2}&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;{P8, P5/2}&lt;br /&gt;

         &lt;td style="text-align: center;"&gt;{P8/2, P4/2}&lt;br /&gt;

Removing the ups and downs from an enharmonic interval makes a &amp;quot;bare&amp;quot; enharmonic, a conventional interval which vanishes in certain edos. For example, {P8/2, P5}'s enharmonic interval is ^^d2, the bare enharmonic is d2, and d2 vanishes in 12-edo. Every rank-2 temperament has a &amp;quot;sweet spot&amp;quot; for tuning the 5th, usually a narrow range of about 5-10¢. 12-edo's fifth is the &amp;quot;tipping point&amp;quot;: if the temperament's 5th is flatter than 12-edo's, d2 is ascending, and if it's sharper, it's descending. The ups and downs are meant to indicate that the enharmonic interval vanishes. Thus if d2 is ascending, it should be downed, and if it's descending, upped. Therefore &lt;u&gt;&lt;strong&gt;ups and downs may need to be swapped, depending on the size of the 5th&lt;/strong&gt;&lt;/u&gt; in the particular rank-2 tuning you are using. In the above table, this is shown explicitly for {P8/2, P5}, and implied for all the other pergens. In the table, the other pergens' enharmonic intervals are upped or downed as if the 5th were just.&lt;br /&gt;

[&lt;em&gt;Question: What to do if the tipping point falls in the sweet spot? Example?&lt;/em&gt;]&lt;br /&gt;