Modal Harmony - yes , the Modes again

What you're seeing is accurate. It's also useless and misleading :)

If you take a scale and harmonize it in four parts, you get the seventh chords that naturally occur from that scale. For C major, those chords would be:

Cmaj7-Dm7-Em7-Fmaj7-G7-Am7-Bm7b5

If you take any "related" mode, you have the same notes... so you get the same chords, but in a different order. D Dorian would give you:

Dm7-Em7-Fmaj7-G7-Am7-Bm7b5-Cmaj7

That's the accurate part. It's misleading, because you can't simply play D Dorian over any of those chords - once you pop in the C7 (or the Bm7b5) you establish a harmonic tension that wants release... and since that release wants to go to a C-root chord, you'll be fighting against the modal melody; it'll sound like C major.

Which makes it useless.

You can use more than two chords, though. You just have to be careful with chords containing the tritone which, as NoteBoat points out, causes an expectation to resolve to a chord other than the tonic, which, in turn, may create an unintended new tonic in the mind of the listener if followed through, or an anti-climax if not. NoteBoat, you meant to say "pop in the G7", not C7 (the curse of early morning postings, eh? :D )

Modes are a purely melodic, pre-tonal concept, and only in the Ionian/Major mode, will the "dominant 7th" resolve naturally to the tonic "key" chord. All other modes either won't work, or else need a bit of help in the form of chromatic alteration (e.g., the harmonic minor scale). But as soon as you start modifying a mode, then, strictly speaking, you're no longer in it.

You'd also want to be careful of heavy use of other 7ths or similarly dissonant chords as they are likely to mask the effect of the mode in use, as you suggested - but you can use more than two chords.

Yeah, that C7 was clearly too early in the day :) (In my defense, I'm always sleep deprived on Saturday mornings!)

Fretsource is right about two chords - I'm thinking the two chords comes from my earlier comments on how they're typically used (like in Miles Davis' "So What")

But you can use a mode over a chord progression containing a 7th chord, and still have a modal piece. The trick is to recognize what the 7th chord does: it identifies a tonal center for the chord progression through its resolution. So if your chord progression includes a G7 chord, or a Bm7b5 (which also has a dominant function - it's the top four notes of a G9 chord), you're pointing to C as the tonal center.

So use a mode centered on C. For example, if you use C Lydian (C-D-E-F#-G-A-B-C) you'll have a clash with the F in the G7 chord... but that clash will come right at the point of the greatest harmonic tension. If you're careful, you can get away with it. it's even easier in minor keys - let's say you're in a straight i-iv-V in A minor - you could use A Dorian (A-B-C-D-E-F#-G) or A Phrygian (A-Bb-C-D-E-F-G). Phrygian gives you a clash against the V, which highlights the tension. Dorian a clash against the iv... but it's a clash we're already used to hearing - because the melodic minor scale has the same F#.

Because using a mode in "parallel" - i.e. one with the same root as the chord progression - can work over familiar progressions, it's the way I introduce them to the 2% or so of my students who can benefit from them because of the types of music they play. But it also shows the futility of "harmonizing" a mode - when you use a mode in parallel, the chords don't come from the mode; if you use chords that you can derive from the mode, and you include the dominant 7th, the mode doesn't work!

Excellent Noteboat! Now everything's more clear now.
Noteboat , Fretsource thanks again .You're the men !

After reading the excellent input from Noteboat and Fretsource in the Minor Scales harmnoy thread (below) I wanted to bring back this subject.

http://forums.guitarnoise.com/viewtopic.php?f=12&t=44972

Noteboat mentioned that you can use modal chord progressions with more than 2 or 3 chords and stay modal.
I will appreciate if you can give me a couple examples in let's say dorian or phrygian (esp if it has a 7th chord there), because again I find all these material of 'harmonizing modes' which are confusing and misleading.

Well, the modal music that I'm most familiar with is Renaissance and folk music. Harmony in modal Renaissance music is relatively simple as it was still in quite an early stage of development. The trouble is, it couldn't advance much further because of the problem mentioned above - i.e., the tritone that occurs in 7th chords wants to resolve to a tonal centre that in most cases wasn't the mode's tonal centre.
In fact, only the Ionian mode has the tritone in the right place. It occurs between the 4th &7th degrees, and is ideally placed to resolve to, and strengthen, the tonal centre of the mode.
As a result, the other modes died out from mainstream composition, and the modal system was gradually replaced by our present major - minor key system, comprising the old Ionian mode (renamed the major scale) for major keys and the new improved version of the old Aeolian mode for minor keys - (improved by introducing the harmonic and melodic forms of it and calling them the minor scales).

Modal folk melodies being simple in nature also tend to use simple harmony of a few chords without dominant 7ths so as not to contradict the key centre. A well known example being Scarborough Fair which is a Dorian mode melody, usually harmonised with about 5 chords, e.g., in D minor with chords Dm, F, G, C & Am

Modes were revived in Jazz centuries after their initial demise, and I'll leave it to NoteBoat or others to supply info on how they're used - as far as I know modal jazz harmony is also kept simple to preserve the effect of the mode, I guess.

Yeah, jazz keeps the harmony simple on modal tunes - just vamps, avoiding the V chord (as well as any other chord that harmonizes in four voices to a dominant chord type).

For songs that use a V chord, you'd have to look to folk music - Scarborough Fair is as good an example as any. With the chords Fretsource posted for it, it's in Dm - but it includes A, the dominant chord (but played as a simple triad)

For songs that use a V chord, you'd have to look to folk music - Scarborough Fair is as good an example as any. With the chords Fretsource posted for it, it's in Dm - but it includes A, the dominant chord (but played as a simple triad)

Ah - the Harmonic Dorian mode. :D Ok - I just made that up, but it's a good example of how the modes gave way to the major-minor system. If you harmonise it with A major instead of A minor, the note C# makes it sound more like a straightforward D minor key song (using the harmonic/melodic form) and less Dorian.

It can be harmonised equally well with A minor for a more 'olde worlde' modal sound, or with A major for a more modern and perhaps more familiar minor key sound.

Yes, exactly - the harmonic Dorian! You just take the Dorian and raise the 7th... on, wait, that's the jazz minor. Or is it the ascending melodic minor? Or maybe it's the major b3? Or am I getting all of this confused with the harmonic phrygian #2???

:)