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28/27
|2 -3 0 1>
62.9609 cents
sound sample

The septimal chroma or septimal third-tone is the superparticular interval 28/27, which has the seventh triangular number as a numerator and which is the difference between 15/14 and 10/9, 9/8 and 7/6, 9/7 and 4/3, 3/2 and 14/9, 12/7 and 16/9 and 9/5 and 28/25.

http://en.wikipedia.org/wiki/Septimal_third-tone