22/21
22/21 |1 -1 0 -1 1>
80.537 cents
22/21 is a small superparticular semitone of about 80.5¢ that appears in 11-limit Just Intonation. It makes the difference between the 21st and 22nd overtone. It is close in size to 21/20, a 7-limit superparticular interval, differing from it by 441/440, about 3.9¢.
The single degree of 88cET can function as both 21/20 and 22/21, thus tempering out 441/440.
See: Gallery of Just Intervals, List of Superparticular Intervals