What are diatonic scales in music?
Most musicians and music students are familiar with the term, 'diatonic scale', but many aren't sure what it means. Books of scales show lots of scale types for readers to practise, such as major, harmonic minor, pentatonic minor, blues, etc., but there's never any diatonic scale mentioned.
The reason is simple. It's a class of scale, not an actual scale. A scale is classed as diatonic if it satisfies all of the following conditions:
- It has seven notes (or eight counting the octave note at the end).
- The notes are separated by a mixture of five whole tones (whole steps) and two semitones (half steps).
- The semitones are separated from each other by either two or three whole tones
The seven diatonic scales
There are, in fact, seven possible configurations, meaning that there are seven types of scale that can be classed as diatonic. These include four medieval scales known as the 'church' or 'ecclesiastical' modes named: Dorian, Phrygian, Lydian, and Mixolydian, plus two Renaissance Period newcomers, called the Ionian and Aeolian modes (which later still, became known as the major and natural minor scales, respectively). Last (and most definitely least), came the Locrian mode, which was considered a purely theoretical scale with no practical use in music due to its awkward interval structure.
Note* Each of the seven modes, had a so-called plagal version,with the prefix 'hypo' added, e.g., the Hypodorian mode. These were just the same modes but starting a 4th lower and don't need to be considered as independent scales.
Diatonic scales compared
Transposed to A
Aeolian Mode/ natural minor scale
A B C D E F G A
A Bb C D Eb F G A
Ionian mode/ Major scale
A B C# D E F# G# A
A B C D E F# G A
A Bb C D E F G A
A B C# D# E F# G# A
A B C# D E F# G A
As sharps and flats weren't part of the music system when these scales were first used, each scale could only begin on a specific note of the seven so-called 'natural' notes available at the time, A, B, C, D, E, F & G. First in use were the Dorian mode, which began on D, the Phrygian mode, which started on E, the Lydian mode on F and the Mixolydian mode on G. Many centuries later, the Ionian and Aeolian modes came along, starting on C and A respectively. Shortly after that, to complete the whole diatonic note set, the Locrian mode, starting on B was added.
The first column of the table above shows how the scales are just 'rotations' of each other. It's obvious that with only seven different notes per octave available, there can only be seven ways to arrange them in order of pitch, i.e., seven scales.
Now that sharps and flats have been introduced into music, we can transpose the scales to begin on any note. If we transpose them all to begin on the same note, we can see how each scale's unique series of tones and semitones produces a unique series of notes. In the last column of the table above, they've all been arranged to begin on the note 'A'.
Why are they called diatonic scales?
The term, diatonic means 'through the tones'. Dia is Greek and means through (not to be confused with di meaning two). Tonic refers to the tones. It doesn't refer to our modern use of the word, tonic to mean a key centre. That kind of tonic came many centuries later.
The original use of the term referred to an ancient Greek four string musical instrument tuning arrangement, called the diatonic tetrachord (from tetra meaning 4 and chord meaning string).The intervals between the strings were two whole tones and one semitone, spanning a perfect 4th between the outermost strings. By joining two diatonic tetrachords together (either directly or separated by a whole tone) seven different 'diatonic' scales of eight notes per octave could be formed as shown above.
The Major - Minor Key System
Major and minor scales and tonality
The most commonly used diatonic scale for composing music nowadays, and for the last three hundred years or so, is the major scale, with the minor scale not too far behind.
As sharps and flats finally became fully available, and harmony became increasingly complex and functional, a new 'key' based style of composition known as tonal music was emerging. The distinguishing feature of tonal music is the strong feeling of 'homecoming' achieved by arranging notes and chords in certain ways that emphasise the first scale note (the tonic) as the most important note.
Most modes weren't really suitable for establishing strong 'key centres' or tonal centres or 'tonics' as they're now known. As a result, most of the modes fell into decline, marking the end of the modal period of music around the mid 17th century. Two newly named scales, exactly the same as the Ionian and Aeolian modes, emerged. They were called the major scale and minor scale, respectively, and were the principal note sources of the 'new and improved' major-minor key system of tonal music that gradually replaced the old modal system. Music composed from modes (or at least influenced by them) is still around, though, in folk music, modal jazz and some rock and pop but less so in modern classical music.
The major scale has become so prominent that, nowadays, it's often seen as the 'parent' scale of the old church modes. It's a modern re-interpretation of modes, although obviously not a strictly accurate one given that most of the modes are far older than their so-called 'parent' major scale.
Harmonic and Melodic minor scales
While the natural minor scale is completely diatonic, there are two modifications of it, called the harmonic and melodic minor scales that, strictly speaking, aren't diatonic as they don't fulfill the conditions mentioned above.
A harmonic minor is: A B C D E F G# A.
The interval between the 6th and 7th notes (F-G#) is larger than a whole tone, and the scale contains three semitones, instead of two.
A melodic minor (ascending form) is A B C D E F# G# A
The semitones (B-C & G#-A) are separated by four whole tones, instead of the maximum three.
These modified minor scales are so much part of our diatonic based major-minor system of music, though, that many music theorists class them as, or at least include them with diatonic scales. Others don't, and it's a source of ongoing academic debate whether to include them or not.