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18/13
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563.3823 cents
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In 13-limit Just Intonation, 18/13 is a low tritone measuring about 563.4¢. It represents the interval between the 13th and 18th overtones of the harmonic series. The low tritone in 17edo approximates 18/13 quite well, while 17edo's approximations of 13/8 and 9/8 are both a little sharp. The inversion of 18/13 is 13/9, a high tritone measuring about 636.6¢; the interval between these two tridecimal (13-based) tritones is 169/162, approximately 73.2¢.

Other just tritones include 7/5, 10/7, 45/32, 64/45, 24/17, 17/12, 23/16, 32/23, and, of course, infinitely many more intervals.

See: Gallery of Just Intervals