MINOR SEVENTH

A 'minor seventh' is the smaller of two commonly occurring musical intervals that span seven diatonic scale degrees. The prefix 'minor' identifies it as being the smaller of the two (by one chromatic semitone), its larger counterpart being a major seventh. The minor seventh is abbreviated as 'm7' and its inversion is the major second. Its most common occurrence is built on the root of the prevailing key's dominant triad, producing the all-important dominant seventh chord.
A minor seventh in just intonation corresponds to a pitch ratio of 16:9 or 9:5, while in an equal tempered tuning it is a ratio of 1:2^10/12 (approximately 1.781), or 1000 cents, 3.910 cents longer than the 16:9 ratio and 17.596 cents shorter than the 9:5 ratio.
A related interval is the "harmonic seventh", with an exact 7:4 ratio (e.g., the three-quarter point of the octave, 1.75), which makes it quasi-harmonically significant. This interval is about 969 cents, or one-third of a semitone shorter than the equal-temperament minor seventh.
The minor seventh is considered a mildly dissonant interval, more dissonant than the thirds and sixths, but considerably less dissonant than the minor second and major seventh.
See also:

★ musical tuning

★ list of meantone intervals