Syntonic comma
The syntonic comma, also known as the comma of Didymus, is a small musical interval in just intonation theory, defined by the frequency ratio of 81:80, which corresponds to approximately 21.5 cents.[1] This interval represents the precise difference between the Pythagorean major third, tuned as 81/64 (about 407.8 cents), and the just major third, tuned as 5/4 (about 386.3 cents).[2] In tuning systems, the syntonic comma plays a central role as the interval that is systematically tempered out to achieve greater consonance in thirds and sixths, particularly in meantone temperaments such as quarter-comma meantone, where each perfect fifth is narrowed by one-fourth of the comma to close the circle of fifths harmoniously.[3] Originating from ancient Greek music theory, it was first explicitly referenced by the theorist Didymus in the 1st century AD through his adoption of the 5/4 ratio for the major third, distinguishing it from the purely fifth-based Pythagorean scale.[4] The comma's resolution allows for the inflection of notes in tetrachords, transforming Pythagorean semitones (256:243) into more consonant just semitones (16:15), and it remains a foundational concept in microtonal and historical performance practices.[3]Definition and Properties
Definition
The syntonic comma is a small musical interval that measures the discrepancy between the Pythagorean major third, expressed as the frequency ratio 81/64, and the just major third, with a ratio of 5/4.[5] This difference arises when constructing intervals using pure fifths versus incorporating the simpler harmonic ratio involving the number 5.[6] The interval itself has a frequency ratio of 81/80.[5] Also referred to as the chromatic diesis, Ptolemaic comma, or Didymian comma, the syntonic comma measures approximately 21.5 cents.[7][8] This size makes it a subtle but noticeable discrepancy, particularly in melodic lines where it can introduce perceptible tension, though it is less prominent in dense harmonic textures.[9] In tuning theory, the syntonic comma plays a fundamental role in addressing inconsistencies between interval constructions, enabling more consonant major triads by highlighting and compensating for the offset between Pythagorean and just intonation approaches to thirds.[5][9]Mathematical Calculation
The syntonic comma arises as the interval between the Pythagorean major third, with ratio \frac{81}{64}, and the just major third, with ratio \frac{5}{4}. To derive its ratio, divide the Pythagorean third by the just third:\frac{81/64}{5/4} = \frac{81}{64} \times \frac{4}{5} = \frac{81}{80}.
This yields the syntonic comma's frequency ratio of \frac{81}{80} \approx 1.0125.[10] Equivalently, the comma can be obtained from the ratio between the two types of major seconds in 5-limit just intonation: the Pythagorean major second \frac{9}{8} and the harmonic major second \frac{10}{9}. Their ratio is
\frac{9/8}{10/9} = \frac{9}{8} \times \frac{9}{10} = \frac{81}{80},
where two \frac{10}{9} intervals produce a just major third of \frac{100}{81}, differing from \frac{5}{4} by the comma.[11] To quantify the comma in cents, use the formula for converting a frequency ratio r to cents: $1200 \times \log_2(r). For r = \frac{81}{80}, first compute the base-2 logarithm:
\log_2\left( \frac{81}{80} \right) = \log_2(81) - \log_2(80) = 4\log_2(3) - (4\log_2(2) + \log_2(5)) = 4\log_2(3) - 4 - \log_2(5),
where \log_2(3) \approx 1.5849625 and \log_2(5) \approx 2.321928. Substituting gives
\log_2\left( \frac{81}{80} \right) \approx 4(1.5849625) - 4 - 2.321928 = 6.33985 - 4 - 2.321928 = 0.017922.
Then,
$1200 \times 0.017922 \approx 21.506
cents.[9][12] In the context of the harmonic series, the syntonic comma emerges from the discrepancy between intervals generated by stacking pure perfect fifths of ratio \frac{3}{2} and those derived from natural harmonic thirds. Four stacked fifths yield \left( \frac{3}{2} \right)^4 = \frac{81}{16}, equivalent to two octaves plus a Pythagorean major third of \frac{81}{64} after reducing by $2^2 = 4. This Pythagorean third exceeds the natural just major third of \frac{5}{4} (from the fifth and third harmonics) by the comma \frac{81}{80}.[13] The prime factorization of the syntonic comma's ratio provides insight into its tuning properties: $81 = 3^4 and $80 = 2^4 \times 5^1, so \frac{81}{80} = 2^{-4} \times 3^{4} \times 5^{-1}. In Monzo arrow notation, using exponents for the primes 2, 3, and 5, this is represented as |-4\ 4\ -1\rangle.[13]