Diatonic Intervals

Diatonic vs. Chromatic Intervals

The term diatonic is used to describe any relationship that occurs within the same scale or key. Conversely, chromatic intervals occur when the two notes of the interval are found from outside the same scale or key.

Using the C major scale as our reference:

  • The interval C - E is diatonic in C major, as both notes are found within the C major scale
  • The interval C - Eb is not diatonic in C major, as Eb is not found in the C major scale

Calculating Intervals

Up until this point, we have been counting out our intervals using half-steps and whole-steps while referencing the chromatic scale. For example, if we give the starting note C a value of zero:

Four half-steps above C is E

4 halfsteps above C is E

Counting half-steps and whole-steps for larger intervals can be difficult and confusing. Also, the chromatic scale does not provide us with any information on the harmonic relationship between two notes.

In order to progress with the study of chords and scales, we must make intervals more identifiable, and we must put them into context with their surrounding notes. We achieve this by using the major scale as a reference and assigning each interval a unique and descriptive name. In other words, the distance is calculated in terms of the relationship of the two notes within a given major scale.

Naming Diatonic Intervals

Intervals are named by counting the distance between two notes with reference to a specific major scale. For example, using the C major scale, C-D-E-F-G-A-B-C, as a reference, the interval C - F is a distance of a 4th. The interval is comprised of the four notes: C-D-E-F.

Four half-steps above C is E

4th note of C major is F

Counting the distance between the notes gives you the numeric value of the interval. However, interval names have a further extension to accurately describe them. The interval C - F is a fourth. What type of 4th is the next step in the correct naming process.

Take the lowest note of the interval and assume this note is the root note of the key. Ask the question, is the upper note found within the notes of the key?

Following on from our example above, the answer is yes. F does belong to C major. C - F is a major interval of a 4th and these two notes are both found in the key of C major.

The prefix 'major indicates that this interval is diatonic to the major scale and the 4th lets us know that the second note of the interval is a distance of four scale degrees from the root note.

In summary, interval names are identified by two main elements:

  1. The degree of the highest note relative to the root note of the major scale
  2. A prefix that describes the interval

The Good the Bad and the Ugly

Music is for listening. This comment seems somewhat needless, although sometimes when we study music theory we lose focus on the fact that music is an art heard through the ears.

Terms such as good, bad, ugly, soft and harsh can all be used to describe how we react to what we hear.

Intervals are said to be consonant due to their simple pitch relationship, resulting in a pleasant sound when they are played together. Conversely, dissonant intervals imply an unstable or unpleasant sound. This can be attributed to tension in a piece of music and is resolved by moving to consonant tones.

Below is a list of diatonic interval names (using the C major scale for reference):

Start Note End Note Interval Name Equiv. Steps
C C Perfect Unison 0
C D Major 2nd 2
C E Major 3rd 4
C F Perfect 4th 5
C G Perfect 5th 7
C A Major 6th 9
C B Major 7th 11
C C Perfect Octave 12

Perfect Intervals

Using the C major scale as a reference, lets revisit the interval of C - F.

F is diatonic to C major and is the fourth note in the scale. However, instead of this interval being called a major 4th, it is accurately called a perfect 4th.

All intervals referenced from the root of a major key are considered major. The prefix Perfect is reserved for intervals that have a high degree of consonance. Major intervals of 4ths, 5ths and octaves are called perfect intervals.