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.