Music Theory/Chord Structures
A chord structure, also called a chord progression or harmonic progression, helps indicate where the melody should go.
The purpose of chord structures
It can be argued that many forms of music are organic, while imposing a rigid "structure" can be hampering. This is a legitimate concern. However, consider that rigid chord structures occur most often in improvisational music (especially jazz), which is usually not rigid in itself. Furthermore, many composers do not consider chord structures much when composing, perhaps trying various things out and just going with what works, or maybe simply having an intuitive understanding of the structures involved. Knowledge of chord structures will help you communicate with other musicians, but is not a must for participating in any kind of jam session. Jamming can begin and end anywhere either with or without structure.
Common chord structures
By far the most common chord structures revolve around, or at least contain, the I, IV, and V chords. This is why it is important to know the Roman Numerals for these scale degrees: they pop up a lot! Many songs use only these three chords (and possibly variations of them). The first we will consider is the classic twelve bar blues based on the 'Blues Scale'.
The twelve bar blues
The twelve bar blues, despite its name, pops up in a variety of genres, from jazz to rock. It has been used in a many great songs, such as the theme from the Batman television show. Its most basic structure is this:
I I I I IV IV I I V7 IV I I
The roman numerals are also used for different songs. e.g E E E E / A A E E / B A E E. This can be used for any chords but you must use the same pattern repeatedly. That is, the first four bars are the I chord, the next two the IV chord, the next two the I chord again, and so on. It begins on the tonic and ends on the tonic, which is common in many progressions. Many chord progressions start at the tonic, move away to somewhere else, only to come back to the tonic. To complicate this, however, twelve bar blues structures sometimes feature a turnaround at the end, meaning the very last bar is a V7 chord, or it has a I chord followed by a V7 chord. Turnarounds usually occur at the end of a verse or some other section in preparation for the next section. Usually the very last bar of a song will not have a turnaround.
In some genres, such as jazz, seventh chords occur more commonly, so the basic twelve-bar blues might become something more like this:
I I I I7 IV IV7 I I7 V7 IV I7 I
It can also be modified further. For instance, the tenth bar (the last occurrence of IV) might become another V7 chord. This alteration was often used when adapting the twelve-bar blues to rock and roll.
In jazz, the II-V-I is probably the most common chord progression. As its name indicates, it goes:
IIm7 V7 Imaj7
As always, the exact type of chords can be modified to give a different feeling, such as in this melodic minor, ascending form only, version:
IIm7b5(9) V7alt Imaj7
Note: Only the IIm7b5(9) and V7alt chords are from melodic minor scales.
The Minor II-V-I
Another commonly used chord progression is the minor ii-V-i. One can derive this from the melodic minor scales shown above, while substituting a IminMaj7 for the IMaj7 chord, or by using three modes from one harmonic minor scale , which produces the following chord progression:
IIm7b5(b9) V7b9(b13) IminMaj7(b6)
Applications of II-V-I's
II-V-Is can be chained together, creating complex progressions. Here's an example:
C Bm7b5 E7 (I II V) Am7 Dm7 G7 (I II V) C (etc...) (I etc..)
An example of complicated progression that can be created this way is the "Coltrane Changes", where the "I" chords move by Major 3rd intervals. Here's a simple example:
Dm7 G7 Cmaj7 (II V I ) F#m7 B7 Emaj7 (II V I ) Bbm7 Eb7 Abmaj7 (II V I ) Dm7 etc... (I etc...)
The way the II-V-I progression works is that first the II chord moves by 4ths upwards, which very often produces interesting results, and then when moving from the V chord to the I chord the 7th of the V chord descends a half step and becomes the 3rd of the I chord.
II-V-I progressions can also be used in other styles, such as classical.
Two-chord structures are much simpler than three-chord structures, of course. However, they are harder to work with! It is more difficult to write an interesting piece with fewer chords, therefore the musicians must get the most they can out of them.
One-chord "structures" are uncommon, but they do exist. For instance, Frere Jacques is a one-chord song because it can be played against a single major chord. More contemporary examples are "On the Road Again" by Canned Heat, numerous rap songs, and so on.