The relationship between two notes played together is an interval.
We measure the interval from the diatonic scale of the lowest note.
The lowest note is the root note.
Say D is the root note. Write the diatonic scale of D. The interval from D to F# is a major 3rd. The interval from D to A is a diatonic 5th. The interval from D to C# is a major 7th.
Why are the 3rd and 7th major?
What is the interval from D to Ab?
Common intervals are measured from the root to the major 7th.
Let us presume D is our root. Here are the common intervals of D.
From D to D through one instance of the diatonic scale is an octave.
The octave cuts in half at the flat 5th. The flat 5th is known as the devil's note and tritone. The devil's note is three tones from the root.
The diatonic 4th and 5th are either side of the tritone.
There is one kind of 4th, a diatonic 4th. The flat 5th is a semitone higher. The major 3rd is a semitone lower. When we talk of a 4th we talk of a diatonic 4th.
Harmony is built in 3rds. The 3rd is major 3rd (from the diatonic scale) or minor 3rd (a semitone under the major 3rd).
There are three types of 7th. The major 7th is the 7th of the diatonic scale. The dominant 7th is a semitone lower than the major 7th. The diminished 7th is two semitones lower than the major 7th.
The diminished 7th is enharmonic to the diatonic 6th.
The diatonic 6th is the only kind of 6th. A dominant 7th is a semitone higher. An augmented 5th is a semitone lower. When we talk of a 6th we talk of a diatonic 6th.
Now you can name the common intervals between any two notes.
Can you see a pattern in the intervals and their inversions?
|Semitone||Tone||Minor 3rd||Major 3rd||Diatonic 4th||Flat 5th / Tritone|
|inverted||Major 7th||Dominant 7th||Diatonic 6th||Augmented 5th||Diatonic 5th||Flat 5th / Tritone|