39/14
39/14 |-1 1 0 -1 0 1>
1773.6567561656 cents
39/14, the 2/3-tone or trienthird (one-third third) interval is a somewhat exotic 13-limit interval which, measuring about 1773.7¢, is almost exactly the tritave inverse of 1/3 of a major third of 5/4. The small comma (5/4)/(14/13)^3 = 10985/10976 is tempered out in such notable EDOs as 46, 84, 94, 103, 111, 121, 130, 224, 270, 494 and 764, leading to a variety of temperaments in which three trienthirds make up a major third.
The trienthird was a favorite interval of Avicenna (Ibn Sina) for his scale constructions, and may be considered a larger size of neutral eleventh (an eleventh between major and minor.)
In 13-limit Just Intonation, 39/14 represents the tritave inverse of the difference in size between the tridecimal minor third of 13/11 and tridecimal supermajor third of 14/11. It is also the difference between 13/10 and 7/5; 13/12 and 7/6; 13/9 and 14/9, and of course 13/8 and 7/4 and the inversions of the above. As it combines the primes 7 and 13, it appears in JI subgroup tunings involving those primes.