21/20

21/20 |-2 1 -1 1>

84.4672 cents

sound sample

A small semitone of about 84.5 cents, 21/20 may be found in 7-limit just intonation as, for example, the difference between 8/7 and 6/5, or between 5/3 and 7/4.

http://en.wikipedia.org/wiki/Septimal_chromatic_semitone