Diatonic chords (basics of music①)

About diatonic chords

Diatonic chords are the chords that are piled up the notes of a specific scale according to certain rules. 'Diatonic' means it's from a specific scale or the key of a song. 'Key' is the note or the chord that the song is centered around.
For example, if the key is 'C', the notes that are used are 'CDEFGAB', and the sequence of the notes is 'W W H W W W H (W=whole tone, H= halftone)', this sequence of the notes is called C major scale. If the sequence of the notes is 'W H W W H W W', that scale is called C minor scale. As a side note, 'the whole tone' is the interval between C and D. 'The half tone' is the interval between E and F. (Please try to confirm it using your keyboard. If there is a black key between two notes, it's the whole tone. If there's not a black key between two notes, it's the halftone.)

To summarize so far, C major scale is the scale that we lined up 'CDEFGAB' at intervals of ' W W H W W W H'. And ‘C major diatonic chords’ is the group of chords that we pile up 7 notes regularly on the 7 root notes from a scale. The root tone could be the central note of the chord’s name. For example, the root tone of 'the C major chord’ is C. 
The rule to pile up notes is ‘skipping over one note’. For example, if the root tone is C, we need to pile up some notes on C skipping over one note. So, ‘E G B’ would be piled up on C. For your information, we can choose how many notes we pile up from 2 and 3 according to songs. If you pile up 2 notes on a root tone, it’s going to be a ‘triad’. And if you pile up 3 notes, it's going to be a 'four-chorded'.

Finally, by naming each chord you made, the diatonic chords will be made perfectly. But before naming it, it’s necessary for us to understand what the ‘degree’ is. The degree is the difference between two pitches(the height of sound), and there are 12 degrees per 1 note on a scale. Let’s start with easier things. The image below indicates 8 types of the C major scale’s degree. For example, the degree between C and E is ‘3rd degree’. The degree between C and G is ‘5th degree’. The root tone is ‘1st degree’.

Then, I’ll subdivide these basic 8 degrees and explain the ‘value’ of them. The 3rd degree is divided into ‘major 3rd’ and ‘minor 3rd’. If there is no halftone between 1st and 3rd, it’s ‘major 3rd’. On the other hand, if there is one halftone, it’s ‘minor 3rd’. The 3rd degree is important because it defines whether its chord is major or minor. The 5th degree is divided into ‘perfect 5th’ and ‘diminish 5th’ and ‘ augment 5th’. If there is one halftone between 1st and 5th, it’s ‘perfect 5th’. If there are two halftones, it’s ‘diminish 5th’. If there is no halftone, it’s ‘augment 5th’. The 5th degree doesn’t have a crucial effect on the sound, but it can stabilize the sound because it goes well with the 1st degree. The 7th chord is divided into ‘major 7th’ and ‘minor 7th’. If there is one halftone between 1st and 7th, it’s major 7th. If there are two, it’s minor 7th. The 7th degree would be added to the triad chord which consists of three primary tones(1st, 3rd, 5th) and has the rule to decorate the sound. So-called ‘jazzy chords’ usually have the 7th degree.

To summarize, there are 1st, 2nd, minor 3rd, major 3rd, 4th, perfect 5th, diminish 5th, augment 5th, 6th, minor 7th, major 7th in the degrees.

Finally, I’ll explain the 2nd/4th/6th degrees. We often call 2nd=9th, 4th=11th, 6th=13th. So I’ll call it so from now. If the 9th degree goes down a halftone, it would be called ‘♭9th’. Going up a whole tone, it’ll be ‘# 9th’. If the 11th degree goes down a halftone, it’ll be ‘♭11th’. Going up a whole tone, it’ll be ‘#11th’. But usually  ‘♭11th’ isn’t used because it’s the same as major 3rd. The 13th degree, if there is one halftone between 1st and 13th, It’ll be ‘♮13th’. If there are two halftones, it’s ‘♭13th’.

I’d like to summarize again(the degrees delimited by “/” would mean the same degree. It’s just the difference in how you see it)

【Types of degrees】
1st(root)  ♭9th  9th  #9th/minor3rd  major3rd  11th  #11th/diminish5th  perfect5th   
 augment5th/♭13th  ♮13th  minor7th  major7th  (8th(octave))

【The primary degree consists of diatonic chords】
1st・・・the root tone
3rd・・・determining major or minor sound
5th・・・stabilizing sound
7th・・・decorating sound

【The degrees used for tension chords】
9th  11th  13th

Below, piling up notes, this is ‘C major diatonic chords’ with chord name.

Firstly, the degree next to notes is ‘3rd’, which means ‘major 3rd’ and its chord would be determined as ‘a type of major chord’. On the other hand, if it’s ‘♭3rd’, its chord would be a type of minor chord. If it’s ‘5th’, it’s ‘perfect 5th’(‘♭5th’→’diminish 5th’, ♯5th→augment 5th). ‘7’ means ‘major 7th’ and ‘♭7th’ means ‘minor 7th’.

‘△’ from C△7 is called ‘major’. C△7 consists of ‘ R 3 5 7’, which means that 
① a type of major chord (3= major 3rd)
② stabilized (5= perfect 5th)
③ decorated by a kind of major tone (7= major 7th)

In this way, I’ll analyze the other chords below.
Dm7 ・・・①minor ②stabilized ③major
Em7 ・・・①minor ②stabilized ③minor
F△7 ・・・①major ②stabilized ③major
G7    ・・・①major ②stabilized ③minor
Am7 ・・・①minor ②stabilized ③minor
Bm7♭5・・・①minor ②unstabilized(♭5th) ③minor

What we need to be careful about is ‘G7’ and ‘Bm7♭5’. G7, despite a type of major chord, it’s decorated by ‘minor 7th’. That’s why this chord is so tense. Bm7♭5 is the only chord used ‘diminish 5th’ and this chord is also tense and sounds unstable. Both are the chords having the function called ‘dominant’ and tense.

Above is the basic explanation of diatonic chords. Thank you.