In music, a septimal chromatic semitone or minor semitone is the interval 21:20 (Play (help·info)). It is about 84.47 cents. The septimal chromatic semitone may be derived from the harmonic series as the interval between the twentieth and twenty-first harmonics.
The septimal chromatic semitone equals a just chromatic semitone plus a septimal semicomma (the interval 126:125).
When added to the 15:14 semitone, the 21:20 semitone and 28:27 semitone produce the 9:8 tone (major tone) and 10:9 tone (minor tone), respectively.