Which chords belong to a mode?

Question

My guitar teacher - in my short student days - told me that each mode, in order to be expressed, have to be played over a certain chord.

That chord can either come from a second guitar, or from one guitar, if you play the chord right before you play the melody.

Ionian - Play Major or Major7
Dorian - Minor or Minor7
Phrygian - Minor or Minor7
Lydian - Major or Major7
Mixolydian - Dom7
Aeolian - Minor or Minor7
Lydian - Minor7b5

But then i watched this video:

But on 1:19, he says that If you play the major scale, your chord progression is this: C Major G Major A Minor F Major

Then on 1:50 he says that if you play the Mixolydian mode, these are the chords available to us:

C Major Bb Major F Major

So how do you know the chord available to you for each chord?

One more question, is this concept called 'Modal Harmonies'?

Answer 1

To try to simplify with an answer- Each mode has a parent key. Let's take C Dorian as a first example. Its parent key is Bb. All the notes found in C Dorian are also in Bb major. So it should come as no surprise that all the chords that fitted with something in Bb, as long as it doesn't modulate, will fit over notes from a song in C Dorian.

Let's take G Mixolydian. Parent key C major. All the chords from C major ( C, Dm, Em, F, G, Am, Bo and their respective 7ths) will fit to parts of a piece in G Mix.

One more: C# Aeolian. Parent key, E major. Thus all the chords from E major will be suitable for use in a C# Aeolian piece. E, F#m, G#m, A, B, C#m, D#o.

See my answer to your other question for a bit more info.

Answer 2

Both of these points are correct, but based on different ideas.

In your first example (from your guitar teacher) you are naming chords which work with the modes from the same root note, e.g. C Ionian works with C Major or C Major7. This works because each note contained in the chord is also contained in the mode. There are also more chord extensions which could colour the chord further and still fit, e.g. C Lydian works with C Major7 #11.

In the second example the chap in the video is naming chords which start on different notes in the mode, so rather than only mentioning chords which have C as a root note, the options are opened up to chords which start on any note within the mode. The full range of triads built from C Mixolydian are:

C

Dmin

Edim

F

Gmin

Amin

Bb

and you could add 7ths to get:

C7

Dmin7

Emin7b5

Fmaj7

Gmin7

Amin7

Bbmaj7

All of these chords comprise entirely of notes from the C Mixolydian mode.

You could also think of the rule numerically (I7, iim, iiidim, etc) and apply it to any Mixolydian mode.

Furthermore, you can relate the rule to other modes, e.g. in the example given you would represent D Aeolian, E Locrian, F Ionian, G Dorian, A Phrygian, Bb Lydian.