Intervals
An interval is the distance in pitch between two notes. Intervals are always measured by the number of letter names from the lower note to the higher both being included in the count. Intervals come in different qualities and size. If the notes are sounded successively, it is a melodic interval. If sounded simultaneously, then it is a harmonic interval.

The smallest interval used in Western music is the half step. A visual
representation of a half step would be the distance between a
consecutive white and black note on the piano. There are two
exceptions to this rule, as two natural half steps occur between the
notes E and F, and B and C.

A whole step is the distance between two consecutive white or black
keys. It is made up of two half steps.
Qualities and Size

Intervals can be described as Major (M), Minor (m), Perfect (P),

Augmented (A), and Diminished (d).

Intervals come in various sizes: Unisons, Seconds, Thirds, Fourths, Fifths,

Sixths, and Sevenths.

2nds, 3rds, 6ths, and 7ths can be found as Major and Minor.

Unisons, 4ths, 5ths, and Octaves are Perfect.
When a major interval is raised by a half step, it becomes augmented.

When a major interval is lowered by a half step, it becomes minor.

When a major interval is lowered by two half steps, it becomes diminished.

When a minor interval is raised by a half step, it becomes major.

When a minor interval is raised by two half steps, it becomes augmented.

When a minor interval is lowered by a half step, it becomes diminished.

When a perfect interval is raised by a half step, it becomes augmented.

When a perfect interval is lowered by a half step, it becomes diminished.
INVERSIONS OF INTERVALS

Intervals can be inverted, which basically means you turn them upside

down. The lower note is raised up an octave so that the top note/bottom

note relationship is reversed. The chart below shows the inversions of

intervals
Major becomes Minor

Minor becomes Major

Perfect remains Perfect

Augmented becomes Diminished

Diminished becomes Augmented

 2 becomes 7

3 becomes 6

4 becomes 5

5 becomes 4

6 becomes 3

7 becomes 2