An interval is the distance between any two notes, such as the distance between B and C. Intervals are named simply by the distance between the two notes, so B and C above it is called a 2nd. B and the D above it would be a 3rd, B and the E above it would be a 4th, and so on.

3rds and 6ths are called “consonant intervals”, while 2nds and 7ths are “dissonant intervals.” If you play them, you will understand why. Intervals of 4ths and 5ths are usually considered harmonic intervals too — technically they are called “perfect intervals” because each note of the interval is in each other’s scale. For example, the distance between F and C above it is a perfect 5th — notice that F is in the scale of C, and C is in the scale of F. Perfect 5ths inverted become perfect 4ths, and perfect 4ths inverted become perfect 5ths.

An interval of a 3rd inverted becomes a 6th, while a 6th inverted becomes a 3rd. Try playing these intervals on your piano, and it will become clear to you.
More about major, minor, and perfect intervals in the next post.