Twelve bar blues

The "twelve bar blues" progression is the main underpinning of the vast majority of blues tunes out there. In order to start playing blues (and to take most of the other blues lessons here), you will want to know what a twelve bar blues progression is. Consists of a I-IV-V chord progression. That is, you pick out the first, fourth and fifth note of the key you want to play in (e.g. A-b-c-D-E-f-g) And then play those notes' major chords.

The most basic form of the blues progression (as shown below in the key of E) is to play twelve bars as follows: I-I-I-I-IV-IV-I-I-V-IV-I-I/V.

That would be to play 4 strums of each chord in a bar.
Bars 1-4 play E
Bars 5&6 play A
Bars 7&8 play E
Bar 9 play B
Bar 10 play A
Bar 11 play E
Bar 12 play E for the first two beats, B for the last two

However Blues tend to be played in 7th chords. So simply play A7 where A is noted, E7 where E is noted, and so on.

Source: http://www.wholenote.com/

See also Bebop blues, minor blues, blues.

Twelve Bar Blues is the most common type of blues progression, and has countless variations. The most basic version is of the form

|| I | I | I | I | IV | IV | I | I | V | IV | I | I ||

where I is any major chord, IV is a major chord a fourth above it and V is a major chord a fifth above it. Although twelve bar blues can be in a minor key, it usually implies a major key. Go to minor twelve bar blues for a discussion of that.

For example, in the key of E, a twelve bar blues progression will be:

|| E | E | E | E | A | A | E | E | B | A | E | E ||

You will hardly ever encounter triads in a blues progression, and all the chords are dominant 7 chords, so the progression is:

|| I7 | I7 | I7 | I7 | IV7 | IV7 | I7 | I7 | V7 | IV7 | I7 | I7 ||

or, in E:

|| E7 | E7 | E7 | E7 | A7 | A7 | E7 | E7 | B7 | A7 | E7 | E7 ||

You will hear this progression on lots of B.B. King, Muddy Waters and other famous blues artists' recordings. What really makes the blues a blues is the characteristic IV in the fifth bar. Without it, and the blues loses all meaning. Of Course I am generalizing here. Take for example, Catfish Blues. It is only one chord throughout. It is still a twelve bar blues. Another important characteristic is the V - IV - I near the end. This is atypical of western music in general, as you will more commonly find the IV - V - I cadence. But enough ranting!

The blues has many variations. The most common one is the following:

|| I7 | IV7 | I7 | I7 | IV7 | IV7 | I7 | I7 | V7 | IV7 | I7 | V7 ||
(|| E7 | A7 | E7 | E7 | A7 | A7 | E7 | E7 | B7 | A7 | E7 | B7 ||)

You will hear this progression just about anywhere too.

Then came bebop, and with it, a new type of blues. The old twelve bar progression just wasn't interesting enough, so there came the bebop blues progression, which is much more interesting harmonically.

Melodically, the twelve bar blues relies heavily on the blues scale, which is the pentatonic minor scale with a #4 added. So on the above progressions, the melody and improvisation are most likely to be in E blues scale.