You probably have already noticed that when it is said about a key only major and minor keys are named. That is because there are no other varieties. As it was referred above, the whole scale used in contemporary music consists of twelve equal halftones (it is called the Chromatic scale when it is played on an instrument). The Diatonicism is a fundamental quality of this twelve half tone system. The melodic and harmonic structure of the Diatonicism is the basis of contemporary music. The principle of the mode diatonicism is that when using only whole and half steps there cannot be two half steps in succession (without chromatisms). Modes which meet this requirement are diatonic. Accordingly modes which do not meet the requirements are non-diatonic. Scales which are the mode basis of music (the Major and the Minor) consist of seven degrees. Obviously five tones of our twelve half tones scale are not enabled, leaving inside this orderly system freedom for melodic and mode variations. If we choose a major tonic centre we will get a major key, if we choose a minor tonic centre we will get a minor key. The formula of a Major scale is whole step - whole step - half step - whole step - whole step - whole step - half step. If we accidentally break the formula while playing the Major scale, that is we play one of five tones which are not enabled, we will immediately hear the breach of the "mode melody" by ear. Our ear perceives this tone as alien for the given melody that is breaking the key melody. The mode is "a pitch system of sounds' collateral subordination based on the logical (subordinate) differentiation of these sounds" (T. S. Bershadskaya, 1978). The structure of a Diatonicism is this logical (subordinate) differentiation and the tonality is only a pitch of a tone. The name of a tonality derived from the mane of its first degree and the mode. For example, the first degree is C, the mode is major, so the key is called C Major; if the first degree is G, the mode is minor, and the key is called Gm (G Minor).
In general the Diatonicism is the mode system which consists of seven septenary modes, where the main are the natural Major and the natural Minor. In major keys the first degree belongs to the natural Major, the second - to the Dorian Minor, the third - to the Phrygian Minor etc… (in line with the diatonic structure) In minor keys all is the same, but a natural Minor is on the first degree there, the Locrian Minor - on the second, the natural Major - on the third, etc…In other words, in relative keys (for example G - Em) we use the same scale with different tonic centers.
To explain the principle of the diatonic structure, we will take a two octave Major scale fingering and examine one octave septenary modes on its base. So we play using only notes of G Major scale from the first degree to the first degree in an octave. Just like this we play from the second degree to the second degree in an octave, from the third to the third, from the fourth to the fourth, from the fifth to the fifth, from the sixth to the sixth and from the seventh to the seventh. So there are seven septenary scales, seven different formulas. Let's analyze the results. For facilitation of this process we will examine received scales beginning from the sixth string and compare these "formulas" with the standard ones (major - WWHWWWH and minor - WHWWHWW).
As a result of the done manipulation we get a harmonious system of diatonic modes. This is so called Phythagorean system. The comparison of "formulas" we convert to the comparison of fingerings. Fingerings are actually "formulas" presented on the guitar neck. So we have:
Three major modes - Ionian (Natural) major,
Lydian major (+4),
Mixolydian major (-7).
Three minor modes - Aeolian (Natural) minor,
Dorian minor (+6),
Phrygian minor (-2),
and one half-diminished mode the Locrian minor (-2), (-5).
Pay attention that figures in brackets denote altered degrees of standard major and minor "formulas". If we rise the fourth degree of a Natural major scale at a half step we will get a Lydian major fingering, if we low the second degree of a Natural minor fingering in a half-step we will get the Phrygian minor fingering.
It should be also mentioned that the Lydian and the Mixolydian majors as well as the Dorian and the Phrygian minor differ from their natural analogs with only one sound, and can serve as expansions (for example) in the mode approach to the improvisation.
Pictures below show two octave fingerings of diatonic modes.
A triad, a seventh chord and a pentatonic are built on base of any mode on the same principles as on base of the Natural major and minor. All diatonic modes are septenary and naturally have a triad, a seventh chord and a pentatonic. Here is shown for clearness the diatonic sequence of "E form" triads in the key of G major (Em). Notice that to play this sequence we use only three different chord fingerings though there are seven chords. This is because alterations which distinguish modes from standard ones don't deal with notes of a triad. In majors these are (+4) and (-7) and in minors - (-2) and (+6). Only a triad of the seventh degree (it has two alterations (-2) and (-5)) influences on the structure of a triad, it lead to the appearance of the third chord fingering. Actually an example is a G major scale played in triads. Or if we start from the Em chord, we will get the E minor scale. Look carefully at the picture, arrows show the succession of chord forms.
The sequence of the seventh chords is given in the same key and with the same chord forms as triads. But here we can see one more chord fingering. The seventh step is lowered in the Mixolydian major and this affect the structure of the seventh chord. So to play a major or a minor scale in seventh chords we need to know four different fingerings. These are four seventh chords we have discussed in the Introduction. Now we understand where these chords come from and why they are of such forms. That is: Xmaj7 - the major seventh chord, Xm7 - the minor seventh chord, X7 - the major minor seventh chord (dominant seventh chord) and Xm7b5 - the half diminished seventh chord.
It seems that the pentatonic was created on principle of absence of degrees with diatonic alterations in its structure. So the fourth and the seventh degrees are absent in the major pentatonic as compared with the Major scale. The second and the sixth degrees are absent in the minor pentatonic as compared with the Minor scale. But we get the minor pentatonic with the lowered fifth degree built on base of the Locrian mode (the seventh diatonic degree) where the second and the fifth degrees are lowered. This led us to only three different fingerings.
So, the Diatonicism contains two tonic centers: one tonic centre of the major inclination and the other of the minor inclination. Thus the key is major if we use the Diatonicism on the basis of the major key centre, and respectively the key is minor if we use the minor key centre. All tonalities have the same structure and fingerings exactly as those mentioned above.
Transferring the discussion into more practical course, we should mention that a musical composition or a song written in one key consists of chords built upon the degrees of that key.
The logic of the musical structures in a key based on three logical in-tonal functions. (T) - The tonal function, (S) - the subdominant function and (D) - the dominant function. Chords of the tonic function have a stable and complete sound. The main chord of the tonic function in the Major is the tonic chord of the first degree I maj7. Chords of the subdominant function also sound stably but the logic of a musical composition demand the further continuation. The main chord of the subdominant function in the Major is the chord of the fourth degree IVmaj7. And chords of the dominant function have unstable sound and demand the further movement of a harmony mainly to the tonic. The main chord of a dominant function is the chord of the fifth degree of the Major scale - the dominant seventh chord V7.
Ionian major (natural) scale two octave
Dorian minor scale two octave
Phrygian minor scale two octave
Lydian major scale two octave
Mixolydian major scale two octave
Aeolian minor (natural) scale two octave
Locrian minor scale two octave
Diatonic triads on the guitar neck
Diatonic seventh chords on the guitar neck
Pentatonic on base of every degree of a Major scale
Pay attention that in the minor key we often use the dominant seventh chord V7 on the fifth degree instead of the minor Vm7 for the stronger inclination to the tonic. Notice also the functionality of degrees set off in red.