Interval
The difference in pitch between two sounds. The name of an interval is determined by the number of letters it includes (counting the one it begins with and the one it ends with). Seconds may be minor (E, F), major (E, F#), augmented (E2, F#). Thirds may be minor (E, G), major (E, G#), diminished (E, G2).
Fourths may be perfect (E, À), augmented (E, A#), diminished (E, A2).
The inversion of an interval produces one of the opposite kind except when it is perfect. Inversion of minor 2d produces major 7th, and since all intervals lie within the octave, and the octave contains twelve half-tones, it follows that an interval and its inversion must together make an octave or twelve half-tones. Intervals are further divided into consonant and dissonant, the consonant into perfect and imperfect. The perfect consonances are the 4th, 5th, and octave. They are called perfect because any alteration of them produces a dissonance. The imperfect consonances are the major and minor 3d and 6th, called imperfect because equally consonant whether major or minor. All other intervals are dissonant, that is, one or both the sounds forming them must move in a certain direction to satisfy the ear. If the dissonant is minor or diminished the sounds must approach each other (except minor 2d); if major or augmented they must separate (except major 7th, which may move either way). Intervals are augmented when greater than major or perfect. Intervals are diminished when less than major or perfect.
The prime or unison is often called an interval and if altered, as, C C#, is called an augmented unison or prime; it is more properly a chromatic semitone. Natural intervals are those found in the major scale.
Chromatic intervals are those found in the harmonic minor scale and in chords that include sounds foreign to the scale or key.