Three Chord Theory...
As soon as you are able to play the :"Fifteen Basic Chords", it will become obvious that some sound better together than others. In any key there are three chords which appear in virtually every basic progression. They will always sound good together, whatever order you put them in. they are called the primary chords and they represent the building blocks of all musical composition. A more in depth study is available in the "Harmony and Form" section in Musical Theory.
You can fine these three chords in any key by looking at the major scale. Take the key of C as an example. The Key of C has no sharps of flats so the notes are C D E F G A B C.
The note C is called the root note and the chord built on the root is C major or the tonic chord. The other 2 primary chords are the 4th and the 5th. Counting up the scale that would be F and G. (note that the root is always counted as 1) These chords are also called the sub-dominant and dominant respectively. In any key these chords have the same relationship to each other and together compromise the "Three Chord Theory". Additional information on "Notes and Scales" can be found in the theory section.
The Roman Numeral System is used in music theory to identify each chord in a given key by a roman Numeral. The first chord, the one built on the root note is I. the second note of the scale is II the third note is III the fourth is IV and so on up to VIII which is the octave.
Each chord also has a name according to its position in the scale starting with the tonic(root), supertonic, Mediant, sub-dominant, Dominant, sub-mediant or relative minor, Seventh or leading tone and the tonic or Octave.
the best way of taking in this information and understanding "Three Chord theory" is to familiarize yourself with the sounds behind the rules. And the only way to do that is to play the chords one after another in various combinations and listen to the effects they create.
The chart below sets out many of the most common I-IV-V chord progressions using the "Fifteen Basic Chords". The I and IV chords in this chart can be major or minor but the V chord or dominant is always major and is usually played as a seventh chord.
Blues Chord Progressions: The Blues is a musical form that is based almost entirely on three chord theory. Although the blues was certainly not analyzed by its creators, its formula has survived to become the structure of Pop, the accepted roots of Jazz and the heart of Rock.
the most common blues pattern is known as Twelve Bar Blues. It gets its name from the fact that it takes "bars" to complete each cycle of the chord progressions. A "bar" is equal to a "measure" in music theory or a count of 1-2-3-4 in common time. The blues is difficult to categorize however. there are many variations on the theme and many ways of arranging the three chords. Sometimes the chords are major, sometimes sevenths, sometime there are only eight bars. The blues is characterized as much by its rhythms and its vocal and lead guitar solos as it is by its chord progressions.
Below are four typical blues chord progressions in the key of E.
The first represents what is usually considered to be the basic twelve bar pattern. the second shows the same progression with sevenths, the third is a typical variation and the last one is an eight bar progression rather than twelve.