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.
Different intervals have different qualities, as shown below:
Interval qualities can be best explained with the aid of a chromatic scale. If you look at the one below, 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.
In summary, here is a list of the distances of all common intervals up to an octave, measured in semitones. Each new interval is one semitone further apart than the previous one. Notice that the interval of an octave is exactly twelve semitones. This is because there are twelve different notes in the chromatic scale. Notice also that the interval which has a distance of six semitones can be called either an augmented 4th or a diminished 5th. This interval is also often called a tritone (6 semitones = 3 tones).
Here are all the common intervals ascending within one octave, starting and ending on the note C.