Many guitar players are familiar with modes, whether they be used in the formation of their solos or as part of a larger musical idea. For those who aren't, however, lets look briefly at what exactly a "mode" is, and how they are conventionally defined.
Essentially, a mode is just like a major/minor scale, but with a different pitch center. This means that instead of using scale degree one as tonic, we use a different scale degree. For instance, if we took our C major scale, which is C D E F G A B C, but used D as tonic, D E F G A B C D, we have created a mode, in this case, D Dorian. Determining which scale degree of the major scale we start on is a very common method of determining mode. Here's a table using this method to determine the mode that we are in.
Scale degree Mode
1 (Do) Ionian (Not used, because its the same as major scale)
2 (Re) Dorian
3 (Mi) Phrygian
4 (Fa) Lydian
5 (Sol) Mixolydian
6 (La) Aeolian (Essentially natural minor scale)
7 (Ti) Locrian
When using this method, naming the scale can be a little bit confusing. We always name our scale by which note is the pitch center, but to figure out the mode, we have to compare it to the major scale with the same notes. So if I have a passage with this pitch inventory: E F# G# A B C# D E, I see that I have the same notes as the A major scale. From there, I have to figure out what scale degree E is in A major. A B C D E, its scale degree 5. Then I consult my handy-dandy chart and determine that it must be E mixolydian. Just to make sure we have this skill down, lets try another.
My pitch inventory is A Bb C D E F G A. Since the scale starts on A, we know that it's going to be A something. To figure out what that something is, we have to figure out which major scale those notes belong to. Since that's F Major, our next step is figuring out what scale degree A is in F major. We get 3, which, according to our chart, is Phrygian. This means that the above mode is A Phyrigian.
Just for kicks, lets try one just like you'd see it in context, where the notes are not in nice scalar order. Our melodic line looks like this D E F E C D A C D E F E C D B C A. Pretty complicated, and it almost looks like gibberish. To start getting at this, the first thing we have to do is figure out which notes are actually there, without repeats. So lets go through the line again, and I'll highlight every new note when we get there. (D) (E) (F) E (C) D (A) C D E F E C D C (B) A. Phewph! And when we take out the repeated notes, we get this. D E F C A B. Our next step is to put these in some kind of order. Lets assume that harmonically we can tell that D is tonic, (Don't worry, I'll teach you later how to do that), which means we should start with D. From there its a simple matter of putting these guys into alphabetical order. D E F A B C. Wait a minute! There's no G! What are we gonna do? Well, just like in your English class, we're gonna use our context clues to figure out whether it should be G# Gb or G natural. Well, for it to be G#, according to key, we'd have to have a F# and C# as well, and we have neither of these. For there to be a Gb, we have to have B, E, A, and D flat, and we have none of these. Therefore, the missing note must be G natural, which means we have D E F G A B C D. Notice that these notes are all natural, which means it contains the same pitches of the C major scale, and since it starts on the second scale degree of that scale, we know that its D Dorian.
Again, your steps:
1. Create a pitch inventory of the notes used.
2. Exclude repeated notes.
3. Determine tonic and alphabetize notes.
4. Figure out which notes are omitted and, if possible, what they should be.
5. Figure out which major scale the given notes belong in.
6. Figure out which scale degree is tonic.
7. Consult chart to name the mode.
That's a bunch of steps to accomplish a pretty simple goal! Let's see if we can't shave a couple steps off with method number two!
Method number two focuses on which notes of a given scale are altered to form a mode. For example, the Lydian mode, which is the mode created off scale degree 4 of the major scale, is essentially a major scale with a raised fourth. And Dorian is just like a natural minor scale with a raised sixth. Modes are typically characterized as "major sounding" or "minor sounding," and by using this as well as the altered notes, we can easily determine mode. Here's how it works:
Mode: Characteristic
Ionian Major Scale
Dorian Nat. minor with raised sixth.
Phrygian Nat. minor with lowered second.
Lydian Major scale with raised fourth.
Mixolydian Major scale with lowered seventh.
Aeolian Natural minor
Locrian Minor scale with lowered second and lowered fifth.
By this method, after I develop my pitch inventory and order it and all that jazz, all I have to do is figure out how its different than the major or minor scale. For example, my pitch inventory is A B C# D# E F# G# A. This is just like A major, but with a raised fourth, so it must be A Lydian. Again, our pitch inventory is F Gb Ab C Db Eb F. This is just like F minor with a lowered second, so it must be F Phrygian. This method is, in my opinion, much simpler than the method previous. Here are your steps for that one.
1. Create a pitch inventory of the notes used.
2. Exclude repeated notes.
3. Determine tonic and alphabetize notes.
4. Figure out which notes are omitted and, if possible, what they should be.
5. Determine how this pitch inventory is altered from the major or minor scale starting at that note, then name the scale.
Now that we know a little bit about modes and how to recognize and form them, lets talk about writing it with them. Its always been my belief that an understanding of melody should come before an understanding of harmony, so lets briefly discuss melodic considerations of modal music.
The most important thing is that if you decide to write in a mode, you need to convey to your listener that you are in fact writing modally. This sounds like a no-brainer, but its very easy to forget. There are three very important ways to impart your intention of modal writing onto your listener. The first is by making sure you use the tonic of your mode as tonic. I.e, if you're writing in F Lydian, F needs to be your tonic. It will be very tempting to turn C into tonic, because it sounds right in your head, but if you do that, you've stopped writing modally and have returned to plain old C Major. The second common way mode is obscured is by lowering/raising the altered tones back to their original major/minor state. Again, using our example of F lydian, if we take that raised fourth, B natural, and continually use Bb, we've once again gone back to major tonality, and stopped writing in mode. The final way to emphasize mode is by emphasizing the distinctive tones/intervals of the mode. For a third time using our example of F Lydian, there's no better way to remind us that we're in Lydian than by using that B natural in key melodic points, such as the strong beats of a measure, or to set up a resolution, or anywhere where it can be prominent or heard. By the same token, that distinctive tritone between tonic and subdominant, or scale degrees one and four, should be used in much the same manner. Aside from these, the rules for modal melody are much the same as those for conventional melody, maintain a unique high/low point, balance melodic leaps, etc. I would direct you to my lesson on melody if you have any questions, or PM me, I'd be happy to answer your questions.
Now, on to modal harmony. This is a very interesting subject, mostly because the possibilities are so vast. Before we talk about some of the rules, I'd like to offer the disclaimer that you can take a lot, I mean a LOT of creative liberties with these ideas. These are only very loose guidelines, and I'd encourage you to experiment with some chromaticism or what have you in your harmony to spice up some of these progressions. If it sounds good, more power to you!
Anyway, as I said in the melody section, a good modal practice is to emphasize the color tones of a mode. This also goes for harmony. For example, in the Mixolydian mode, I'd use a lot of the I7 chord, or 1, 3, 5 b7. This is a harmony that is not found diatonically in the major or minor scale at that scale degree, which makes it unique and interesting. This becomes much more prevalent when we get into extended chords, such as the Lydian Imaj711 chord, or, in F Lydian, F A C E G B. Notice that B is not diatonic to the key of F major, and if were to write this chord in F major, we would get F A C E G Bb. These kind of extended chords are very common in jazz harmonies, as well as 20th century instrumental music. Now, this means that we'll occasionally get minor v chords (Aeolian), Major VII chords (Mixolydian), and diminished io chords (Locrian). These are just some of the interesting harmonic textures that occur in modal harmony, and using these can establish your sense of modality in your song.
Now, one important thing to think about is the ability to resolve modal ideas, because a lot of times we lack that 7-1 pull that is so important to Western music. Well, there's a couple of ways to get around this, and its up to you to determine which one sounds best and which you'd like to use at the particular moment. Again remember, the possibilities are endless.
The first way around this problem is just by ignoring it. So you have a minor v chord, who cares? So that 7-1 is a whole step and not a half step. Oh well! You picked the mode for a reason, right? You embrace the idiosyncrasies of the mode, and you don't want that 7-1 leading tone resolution that the rest of the world seems to crave. This is probably the most simple, because it continues to use the same chord progressions you're familiar with in major/minor tonalities, as well as completely diatonic harmony.
Another option is to change the harmony to that which we are used to, and raise scale degree seven to make V major. The problem with this is that it can very easily obscure the mode, and while it sounds "right", it becomes inherently less modal. But, when a very strong resolution is needed, and with extreme discretion, it is an option.
The third option is by avoiding the 7-1 completely and using other, less common ways to resolve to tonic. In the case of Phrygian and Locrian modes, we can accomplish this by using the lowered scale degree two as a "reverse leading-tone." It works just like 7-1, but we move in the opposite direction. There's still a half step, and the pull still exists, but we're much less accustomed to it in this form. But, we can use harmonies such as the ii7 chord to set up that resolution. We can also use the color tones as the scales as a means to move towards tonic, such as the raised sixth in Dorian to use the Major IV as sort of a dominant chord to create a plagal, or IV-i cadence. In Mixolydian, we can use the ii chord as another form of plagal cadence to set up tonic. Again, the possibilities are endless.
The fourth option, if all else fails, is what I like to call "tonic by brute force." In this method, we establish tonic by shoving that tonic chord in as many important places as we can so that the listener knows that the chord is what we intend to be tonic. Even if the chords preceding it do not nicely set up a resolution, we still cram that tonic harmony in there, so that its "tonic."
I promised that I'd teach you how to determine what note is tonic if you're hearing modal music, and this last method is really the crux of the theory. You have to listen to where you think the important chords are, where the resolutions are, and figure out which note is the root of the chord. This is not an exact science, and it takes some practice, but it really is the best way to determine what goes where.
Before I part with you for a time, I'd like to again remind you that the above lesson is by no means a be-all-end-all guide to modal harmony. There are about a gillion and a half options open to you for using your modal harmony, but this lesson should serve as a beginning, cursory guide to modal harmony.
Enjoy!
-John
/ my profile 
