To augment an interval is to increase it in size by a half-step. With unisons, 4ths, 5ths, and octaves, the augmented interval is one half-step larger than the corresponding perfect interval. With 2nds, 3rds, 6ths, and 7ths, the augmented interval is one half-step larger than the corresponding major interval. The symbol for an augmented interval is a plus sign, placed in front of the interval number. For example, +4th designates an augmented 4th, a half-step wider than a P4th, or perfect 4th.
Augmented interval
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