In music theory and musical tuning the Holdrian comma, also called Holder's comma, and sometimes the Arabian comma, is a small musical interval of approximately 22.6415 cents, equal to one step of 53 equal temperament, or ( play (help·info)). The name comma is misleading, since this interval is an irrational number and does not describe the compromise between intervals of any tuning system; it assumes this name because it is an approximation of the syntonic comma (21.51 cents)( play (help·info)), which was widely used as a measurement of tuning in William Holder's time.
The origin of Holder's comma resides in the fact that the Ancient Greeks (or at least Boethius) believed that in the Pythagorean 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 of 5 tones + 2 diatonic semitones, 5 x 9 + 2 x 4 = 53 equal commas. Holder attributes the division of the octave in 53 equal parts to Nicolas Mercator, who would have named the 1/53 part of the octave the "Artificial Comma".
Mercator's comma and the Holdrian comma
Mercator applied logarithms to determine that (≈ 21.8182 cents) was nearly equivalent to a syntonic comma of ≈ 21.5063 cents (a feature of the prevalent meantone temperament of the time). He also considered that an "artificial comma" of might be useful, because 31 octaves could be practically approximated by a cycle of 53 just fifths. William Holder, for whom the Holdrian comma is named, favored this latter unit because the intervals of 53 equal temperament are closer to Pythagorean tuning than that of 55. Thus Mercator's comma and the Holdrian comma are two distinct but related intervals.
The name "Arabian comma" may be inaccurate; the comma has been employed mainly in Turkish music theory by Kemal Ilerici, and by the Turkish composer Erol Sayan. The name of this comma is "Holder koması" in Turkish.
c d e f g a b c' commas: 9 8* 5* 9 9 8 5 (In common Arabic and Turkish practice, the third note in rast is almost exactly halfway between a western Major and minor third, often referred to a "neutral third" by musicologists)
while in contrast, the makam nihavend (similar to the Western minor scale):
c d e♭ f g a♭ b♭ c' commas: 9 4 9 9 4 9 9
- Habib Hassan Touma (1996). The Music of the Arabs, p.23. trans. Laurie Schwartz. Portland, Oregon: Amadeus Press. ISBN 0-931340-88-8.
- A. M. S. Boethius, De institutione musica, Book 3, Chap. 8. According to Boethius, Pythagoras' disciple Philolaos would have said that the tone consisted in two diatonic semitones and a comma; the diatonic semitone consisted in two diaschismata, each formed of two commas. See J. Murray Barbour, Tuning and Temperament: A Historical Survey, 1951, p. 123
- W. Holder, A Treatise of the Natural Grounds, and Principles of Harmony, London, 3d edition, 1731, p. 79.
- "The late Nicholas Mercator, a Modest Person, and a Learned and Judicious Mathematician, in a Manuscript of his, of which I have had a Sight."
- W. Holder, A Treatise..., ibid., writes that Mersenne had calculated 58¼ commas in the octave; Mercator "working by the Logarithms, finds out but 55, and a little more."
- Holder, William, A Treatise on the Natural Grounds, and Principles of Harmony, facsimile of the 1694 edition, Broude Brothers, New York, 1967. (Original pp. 103–106.)
- Monzo, Joe (2005). "Mercator's Comma", Tonalsoft.