To diminsh an interval is to decrease it in size by a half-step. With unisons, 4ths, 5ths, and octaves, the diminished interval is one half-step smaller than the corresponding perfect interval. With 2nds, 3rds, 6ths, and 7ths, the diminished interval is one half-step smaller than the corresponding minor interval. The symbol for a diminished interval is a small circle, placed in front of the interval number. For example, o4th designates a diminished 4th, one half-step narrower than a P4th, or perfect 4th.
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