The diatonic scale is most familiar as the major scale or the "natural" minor scale (or aeolian mode).

The diatonic scale is a very important scale. Out of all the possible seven note scales it has the highest number of consonant intervals, and the greatest number of major and minor triads. The diatonic scale has six major or minor triads, while all of the remaining prime scales (the harmonic minor, the harmonic major, the melodic and the double harmonic) have just four major or minor triads.

The diatonic scale is the only seven note scale that has just one tritone (augmented fourth/diminished fifth). All other scales have two, or more, tritones.

The diatonic scale and the melodic scale are the only scales which have just two types of second - the major second and minor second (represented by a whole tone and a semitone, respectively). All the other scales also have a step size of an augmented second as well (represented by a three semitone step), giving them three consecutive-step sizes.

The diatonic scale is also a proper mode. In fact all of the prime scales with the exception of the double harmonic scales are proper modes.

The diatonic scale is therefore an ideal resource for both melodic and harmonic music - it has lots of consonant triads, it has few dissonant intervals, and it is melodically smooth with just two consecutive-step sizes.

The seven modes of the diatonic scale (Lydian, Ionian, Mixolydian, Dorian, Aeolian, Phrygian and Locrian) were (with the exception of the Locrian) widely used in the pretonal Western music of the Middle Ages.

In a tonal harmonic music system, however, only two of these modes are effective - the major scale (Ionian) and the aeolian mode.

The major scale and its triads can be represented numerically as:

So if the tonic is c, the scale will consist of the following notes and triads:

There are semitones between the 3rd and 4th degrees and between the 7th and 8th degrees, and there are wholetones between all the other adjacent notes. Using this pattern, a major scale can be built on any starting note.

The major scale is very effective at producing a strong tonic function on the major chord built on its home note. The only other chord which is capable of exerting any tonic function is the vi chord. This chord is often used as the final in a deceptive cadence. It brings resolution, but the feeling is still there that a full resolution has not occurred and that that is only achieved with the arrival of I.

If the vi chord did begin to sound like the full resolution, and none of the scale notes has been altered, then we have modulated to the relative aeolian mode. Modulation to the relative aeolian mode usually occurs through the use of the (major scale) iii chord. This chord tends to push the listener to reinterpret the major scale as its relative aeolian, and to interpret what was the vi chord (and is now the i chord) as the full tonic. It is for this very reason that the iii chord (or mediant as it is known in many harmony textbooks) has to be introduced so judicially and with great care to the effect it is having. Generally speaking, the iii chord should be used only as the harmony behind a descending scale line of 8, 7, 6 (harmonised with, respectively I, iii, IV), or it should resolve directly to vi before subsequently moving to a full cadence to I. Most importantly, it is safer not to follow or precede iii with ii.

The tendency of the diatonic scale is toward the major interpretation rather than the aeolian, so once we are in the aeolian mode it is quite natural to drift back to its relative major scale.

Apart from the iii chord, there are no other limitations as to the use of the chords - they can be introduced freely, at any time and in any order. Below I will be looking at some of the more conventional three chord progressions that function as cadences in the major scale.

IV - V - I and ii - V - I are the classic cadential chord progressions. They are probably the most decisive of all harmonic cadences, and firmly establish the I chord as a chord of rest and stability, and the root note of that chord as the home note of the key.

The strength of the cadence is affected by whether the chords are played in root position or inverted. If all the chords are played in root position the cadence is at its most potent.

If the V is played in first or second inversion the cadence is less decisive.

Ending on I in its first inversion strongly establishes the key, but the first inversion of the tonic is an unstable structure in itself so this progression does not provide as strong a point of rest. To end on the second inversion of I will greatly reduce the stability of the cadence - the second inversion of either the major or the minor triad is so unstable that it demands resolution.

The instability of the second inversion can, however, be used to strengthen the cadence when it is used as a preparation of the dominant chord in the following progression:

• IV - I⁶₄ - V - I
• (F - C⁶₄ - G - C)

This chord progression is probably the most decisive cadential formula, and it is an extremely familiar device used to give a feeling of completion and resolution. In this context the second inversion of the tonic can be best understood as a decoration and extension of the dominant chord.

The aeolian mode and its triads can be represented numerically as:

1	2	3	4	5	6	7	Notes
i	ii0	III	iv	v	VI	VII	Chords

So if the tonic is c, the scale will consist of the following notes and triads:

c	d	e	f	g	a	b	Hear these notes
c	d0	E	f	g	A	B	Hear these chords

There are semitones between the 2nd and 3rd degrees and the 5th and 6th degrees, with wholetones between all other consecutive degrees. Using this formula the aeolian mode can be built on any starting note.

The modes of the diatonic scale are most commonly found in medieval music and folk music, so using them can suggest a somewhat "ancient" quality.

The aeolian mode is generally avoided in common practice classical music, because its cadences are less decisive than the harmonic minor scale's. But the softer and more relaxed sound of the aeolian tonality, has been used by twentieth century composers, such as Ravel and Vaughan Williams, seeking to expand the conventional tonal resources bequeathed by composers of the nineteenth. It is also found in some modern pop and dance music, where the use of the altered seventh of the harmonic minor scale can sound like a clumsy and unnecessary device.

The aeolian mode can easily drift into its relative major scale. This is best avoided by not using progressions that are reminiscent of the major mode cadences (such as aeolian: VI - VII - III which is equivalent to major: IV - V - I), and through frequent use of the v preceding or proceeding iv or VI.

The aeolian mode cadences are much softer than those found in the harmonic minor scale. They are less decisive, but they have an understated, calm and relaxed quality which can be both effective and appealing.

The short piece below uses a chord progression which harmonizes an a aeolian melody. The chords are:

i | v | VII | i | VI | VII | iv  v | i :|

a | e | G | a | F | G | d  e | a :|

and the piece is called Aeolian Wave (midi).