Intervals · Music theory
Number and quality: major, minor, perfect, augmented and diminished, and how to classify any interval.
What an interval is
An interval is the distance in pitch between two notes. If they sound one after the other it is melodic, ascending or descending; if they sound together, harmonic.
An interval is named with two pieces of information: its number (2nd, 3rd, 4th…) and its quality (major, minor, perfect, augmented or diminished). So, “major 3rd” or “perfect 5th”.
A descending melodic interval is named like the ascending one with the same notes: G–C going down is a perfect 5th, like C–G going up. The direction is stated separately: “descending perfect 5th”.
Other ways of classifying them
Besides melodic or harmonic, a melodic interval can be conjunct, joining two neighboring degrees of a scale (a 2nd), or disjunct, skipping some (a 3rd or more). A melody usually mixes steps with an occasional leap, and that mix gives it much of its character.
An interval is also called diatonic when both its notes belong to the same key and chromatic when they do not: C–E is in C major, and C–E♭ is not. The distinction depends on the key, because C–E♭ is perfectly diatonic in C minor.