Why are modes so complicated - What's the point of them?
Modes are one of those topics surrounded by confusion. Even some advanced guitarists struggle to grasp them. The main reason for this is quite simple, most tutorials on the subject only tell you how to play them as scale patterns but give no further explanation of their use. Time to unravel the mystery!
Okay, let's quickly skim over the usual stuff first, then we'll get into the nitty gritty. If we take any major scale and use each note as a new starting point then we can create seven different modes of that scale. The order of the modes are Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian and Locrian.
Using the C major scale for example, we can create seven modes of that scale by starting and ending the mode on each note in turn. The notes in the C major scale are C D E F G A B C. Here are the modes that correspond to the C major scale. Note, the first one, Ionian, can be viewed as just another name for the major scale.
Unfortunately that's pretty much where many tutorials stop with modes so it's no surprise that the subject ends up so confusing and begging the question ... "What's the point of modes?".
The key to understanding modes lies in the understanding of "tonal centre." If you don't understand what this is then you won't understand modes. Let's see if we can shed some light on this without getting too bogged down with music theory. Don't skip ahead here, the following is all relevant.
Tonal centre (or key centre) is pretty much what it says. Think about it as the centre of tonality in a piece of music, the main musical note that feels like the common tone that's holding everything together. Imagine a simple twelve bar major blues in the key of C. You wouldn't argue that it's in any other key. C is definitely the home key, your ears are very used to hearing it and you naturally expect to hear some form of C scale used for a guitar solo, riff or melody to play over the top of it. If you played a B major scale over it, you would know it's wrong.
Now, there are a few things that create this tonal centre, one of them is purely expectation! You are so used to hearing things played in a certain way that anything else just sounds wrong. This is an important thing to grasp even though it sounds obvious. What's not so obvious is why, and this is important ... our ears have a kind of short term memory when we listen to a piece of music. This, combined with the fact that we have heard songs based on common chord progressions probably thousands of times, ultimately leads to expectation. Once we expect to hear something, it becomes the only thing that sounds right. In other words, if we want to hear "C" as the tonal centre then it makes it very difficult to make anything else sound right.
What has short term memory got to do with it? Well, think about the common twelve bar I-IV-V chord progression. In the key of C the chords would be C, F and G. What makes C the home key?. Why not F or G? Well, there are a few things at play here. For a start, all three chords belong to the key of C major so that gives them a common tie, there is no other key that they all belong to together (apart from relative minor key but that's not important here). The main thing that really brings the key home is the V-I cadence.
The "five" chord (G in this case) has a strong tendency to want to resolve up to the "one" chord. It is however our short term ear memory that makes this resolution expected. If you just played the G chord on it's own for the entire length of a song, then G would obviously sound like home. We would be in the key of G. Likewise if we played just the F chord throughout the entire length of a piece of music then F would be the home key. Same goes if we just played the C chord alone, we'd be in the key of C. At this point you might be wondering, if that's the case then why is the C F G chord progression all in the key of C and not changing key with each chord?
At what point do these three chords become just the key of C? This can depend on what else is going on in the music. Other instruments and the melody can affect the tonal centre, but short term ear memory and expectation plays a very important part. As the duration of each chord gets shorter, their common ties becomes stronger. If we only changed chord once every two minutes then it would be extremely easy to change key with each chord change. If we change chords every few bars then they start to sound like a "collective" and this influences how our ears interpret it.
In the case of something like a twelve bar blues which we've all heard countless times in one form or another, the expectations become so strong that it's almost impossible to veer away from anything other than what the ear is expecting next, in both chord duration as well as tone.
So how does this help anyone understand modes? The reason is quite simple. Traditional and common explanations of modes only make sense when the scale is played on a single instrument alone, with no accompaniment and starting and finishing on the root notes. In the real world of playing music, it becomes irrelevant because this is not how we play music and it's not how we play with scales on most modern music.