Intervals
An interval is the distance between two musical notes. All melodies and chords are made up of a series of intervals. Intervals are measured in numbers, and are calculated by counting the number of letter names (A B C D E F G A) between and including the notes being measured.
Within an octave, intervals are: unison (two notes of the same pitch played or sung together or consecutively), 2nd, 3rd, 4th, 5th, 6th, 7th and octave (two notes an octave apart). Thus A to B is a 2nd interval, as is B to C, C to D etc. A to C is a 3rd interval, A to D is a 4th, A to E is a 5th, A to F is a 6th, A to G is a 7th and A to the next A is an octave.
Intervals may be melodic (two notes played consecutively) or harmonic (two notes played at the same time). Hence two people singing at the same time are said to be singing in harmony.
Interval Qualities
Different intervals have different qualities, as shown here:
Interval qualities can be best explained with the aid of a chromatic scale. If you look at the following diagram it is easy to see that since intervals are measured in semitones, they may begin or end on a sharp or flat rather than a natural note.
Perfect intervals are 4ths, 5ths and octaves. If you widen a perfect interval by a semitone it becomes augmented (added to). E.g., if you add a semitone to the perfect 4th interval C to F, it becomes the augmented 4th interval C to F#. Notice that the letter name remains the same–it is not referred to as C to Gb.
If you narrow a perfect interval by a semitone it becomes diminished (lessened). E.g., if you lessen the perfect 5th interval D to A by a semitone, it becomes the diminished 5th interval D to Ab. Again, the letter name remains the same–it is not referred to as D to G#.
Major intervals (2nds, 3rds, 6ths and 7ths) become minor if narrowed by a semitone and minor intervals become major if widened by a semitone. A diminished interval can be created by narrowing a perfect or minor interval by a semitone. An augmented interval can be created by widening a perfect or major interval by a semitone.
