Crash Course in Jazz Harmonic Theory

I have ordered "The Jazz Piano Book," and it's estimated delivery time is today, at which point I will hopefully be tackling jazz piano with renewed vigor.  But for now, I guess I would like to pause, and convey a little bit of theory, so that people interested in a jazz theory at a glance can get it, and also as something to refer to.  I suppose I will drop in tidbits on how to read the symbols in fakebooks where I can, but that is not really the focus of this blog post, that would be more of a practical theory type posting.

Also, over the course of writing this, I have realized that I really should not be writing this.  I don't quite know what I am talking about, but I am trying to confine this discussion to things that I believe are fairly common knowledge, even if I can't expound upon them or entirely justify them.

Chords


Chords are often referred to as "vertical", since that is how they appear written out on staff paper.  In Jazz, generally speaking, chords come from the piano.  Of course, you do not play notes abstractly, but rather you play a specific voicing--a layout of notes on the keyboard.  And you do this using a specific fingering, but that is purely a matter of technique, the music produced by the piano does not indicate which fingers struck which keys.

Jazz, at a fundamental level, is concerned with the ii-V7-I progression.  Thus, these different chords represent functions.  The V is often referred to as the dominant, and in a related but not identical concept, all major chords with the flattened seventh are called dominant seventh.  Anyhow, here are the basic notes to these chords:

Note that those are not necessarily the best voicings to use on a piano.  Also note, that in general, you are better off to drop the fifth than the other notes in jazz in the event that you are trying to economize--either because you are playing in a low register and want to avoid mud, or because you are out of fingers, or because you want a sparser sound, etc.  The fifth does not help to determine which of the above three functions a chord is.  

Also note the C delta notation, actually my software is putting that in.  Alternate notation is maj7, and also if you see just a 6 or a 9 that is also going to be a major seventh chord (most likely).  If you see just the letter "C" in a fakebook, that generally means a dominant seventh, whereas a "C7" would probably indicate that more extensions--e.g. a ninth--are supposed to be included as well.  Often context will help, sometimes it will be confusing.  

Scales and Modes

When making melodies designed to fit with a chord progression, or extending harmonies using a sequence of notes over time, you need some theory to help you with the horizontal aspect of harmony.  The main scale in jazz is just the good old fashioned major scale, which we will use in C.  It looks like this: 

Note that that's just all the notes in the key of C, which is the key which results from there being no key signature.  Those notes form the C major scale.  But, taking the c to be the root of the scale, you have the C Ionian mode.  Taking the D as the root of the scale with these notes gives you the D Dorian mode.  Taking the G as the root gives you the G Mixolydian mode.  You can play the C ionian over the I. the D Dorian over the ii, and the G Mixolydian over the V.  You can transpose these scales to another key by moving all of the notes the same interval (i.e. the same number of steps), with the root note being in the desired key.

An astute reader will note that doing that is the same as just playing the C major scale over all three.  And that is true.  However, sometimes you will see a stray dominant seventh in a progression that doesn't seem to link up to anything else (or what amounts to the same, links up chromatically), and in those situations is easiest to think of playing over that chord in the mixolydian mode of that chord.  Furthermore, the fourth of the ionion mode over a major chord can sound a little dissonant, so it might work to play in a lydian mode (the mode you get using the notes of the C major scale starting with F, it is the same only with a sharpened fourth) while you are over the I.  Similarly, the Dorian can work well over the vi chord, which in C would be an Am.  

Additionally, it is helpful to start thinking in terms of modes because there are alternative scales that can be used in many circumstances -- such as the whole tone scale (all notes are one whole step apart) -- but often those are not viable options for an entire piece, and sometimes are just be dropped in for one chord.  

Conclusion

This is not even scratching the surface of the enormous topic that is jazz harmonic theory, and it's a topic that as soon as you pin it down, someone will change it, just because you pinned it down and they've got a horn and want to make you look like an idiot.  Damn jazz people!

But anyhow, it's hopefully a useful introduction to the topic, and gives you something of an idea of what is going on with all those notes and chords.

Feel free to leave questions in the comment sections, or otherwise ask me, but remember I'm not a professional jazz instructor or anything.

Take care,

Malcolm