Intervals are harmonic or melodic.

An Harmonic interval is the difference in pitch between two tones sounding
at the same time.

A Melodic interval is the difference in pitch between two tones sounded in
succession by the same voice. [Fig. 1.]

[Illustration: Fig. 1.]

Harmonic intervals are divided into Consonances and Dissonances.

Consonances are classed as perfect or imperfect.

The Perfect consonances are the Unison, Fifth, and Octave. [Fig. 2_a_.]

The Imperfect consonances are the Major and Minor Thirds and Sixths. [Fig.
2_b_.]

All other intervals are dissonances.

[Illustration: Fig. 2.]

A Diatonic progression is one in which both name and pitch are changed.
[Fig. 3_a_.]

A Chromatic progression is one in which the pitch is changed a semitone,
while the name remains the same. [Fig. 3_b_.]

[Illustration: Fig. 3.]