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[[de:100/99]]
{{Infobox Interval
| Name = Ptolemy's comma, ptolemisma
| Color name = 1uyy1, luyoyo 1sn,<br>Luyoyo comma
| Comma = yes
}}
In [[just intonation]], '''100/99''', the '''Ptolemy's comma''' or '''ptolemisma''', is a [[superparticular]] interval of around 17.4¢ which makes the difference between [[11/10]], the greater undecimal neutral second of about 165¢, and [[10/9]], the 5-limit minor whole tone of about 182.4¢. 

== Tempering ==
Temperaments in which 100/99 is tempered out include [[22edo]], [[41edo]] and others; in such systems, 11/10 and 10/9 are equated. This is in contrast to the more familiar tempering out of [[81/80]], which results in meantone and other temperaments, in which 10/9 is equated, not with 11/10, but with [[9/8]]. However, meantone and ptolemismic are not necessarily mutually exclusive–the [[flattone]] temperament (known as "hypnotone" in the 2.3.5.11 subgroup) merges both and has [[11/10]], [[10/9]] and [[9/8]] all tempered to a 10/9-like interval. Along with being the difference between 11/10 and 10/9, 100/99 is the amount by which an [[11/8]] and two [[6/5]]s fall short of an octave, so that tempering out 100/99 allows for the [[ptolemismic triad]] which has intervals of 11/8, 6/5, 6/5. It is also the difference between [[25/24]] and [[33/32]]. In terms of commatic relations it notably factors into [[225/224]] and [[896/891]].
== Approximation ==
If the interval is not tempered and instead treated as an identity, it is extremely closely represented by one step of [[69edo]]. The associated unnoticeable comma, [[ptolemi-nicema]], represents the difference between 69 100/99s and the octave, is only 0.56 cents wide.

== See also ==
* [[Ptolemismic triad]]
* [[Rank-4 temperament #Ptolemismic (100/99)]]
* [[Ptolemismic clan]], the clan of rank-3 temperaments where it is tempered out
* [[Small comma]]
* [[List of superparticular intervals]]
* [[Gallery of just intervals]]

[[Category:Ptolemismic]]
[[Category:Commas named after polymaths]]

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