What is interval? Interval is the relationship between two notes. It is the distance between two notes. It can also be defined as a situation whereby two notes are sounded or written together. In short, interval has to do with two notes.

Interval could be written vertically or horizontally. It could be sounded simultaneously or successively. When interval is written vertically, it will sound simultaneously. When interval is written horizontally, it will sound successively.

Put it differently in the musical terms, we melodic and harmonic intervals. Interval is said to be melodic when the notes written vertically or sounded simultaneously. On the other hand, interval is said to be harmonic when the notes written horizontally or sounded successively.

Whether it is written vertically or horizontally, sounded simultaneously or successively, the fact is that, it has to do with two notes. Interval is the basis of two-part harmony and counterpoint. The two part harmony is sometimes (especially in choral music) referred to as duet.

Intervals are approached from two perspectives; quantitative and qualitative. Qualitative approach has to do with the use of numbers such as 1, 2, 3, 4, 5, 6, 7, 8. The “1” is referred to as unison. The “2” is referred to as 2^{nd}. The “3” is referred to as 3^{rd}. The “4” is referred to as 5^{th}. The “6” is referred to as 6^{th}. The “7” is referred to as 7^{th}. The “8” is referred to as octave.

Qualitative approach, on the other hand has to do with the use of adjectives such as major, minor, perfect, diminished and augmented to qualify the intervals. At unison (1), 4^{th}, 5^{th}, and octave (8), we have perfect intervals. Hence, we have perfect unison (P1), perfect 4^{th} (P4), perfect 5^{th} (P5), and perfect octave (P8). At 2^{nd}, 3^{rd}, 6^{th}, and 7^{th}, we have major and minor intervals. Hence, we have major and minor 2^{nd} (M2, m2), major and minor 3^{rd} (M3, m3), major and minor 6^{th} (M6, m6), major and minor 7th (M7, m7).

The above intervals are diatonic in nature. Diminished and augmented are chromatic in nature. If any of the above intervals is decreased by a semitone, it will become a diminished interval. Hence, if a perfect 5^{th} (C –G) is reduced by a semitone (C-Gb), it will become a diminished 5^{th}. Similarly, if any of the above intervals is increased by a semitone, it will become an augmented interval. Therefore, if a perfect 5^{th} (C –G) is increased by a semitone (C-G#), it will become an augmented 5^{th}.

Qualitatively, when a major (M) interval is inverted, it becomes a minor (m). When a minor is inverted, it becomes a major. Perfect remains perfect either way. Augmented (A) becomes diminished (d); Diminished becomes augmented.