Playing blues is a great way to bring together many musical ideas and tools. The chord form provides for exploration of extensions and alterations as well as the ideal medium to learn to improvise. Blues, in it's most basic form, exists in 12 measures and is made up of three chords, the I, IV and V. Usually, all three chords are played as dominant 7ths.. The following example is a 12 bar blues in G. G is the key chord so it gets the number I. Counting up 4 from there brings us to C and, one more to 5 brings us to D. All the chords are dominant so: I=G7, IV=C7 and V=D7. The basic blues chord order (progression) is: 4 measures of the I chord, 2 of the IV chord, 2 of the I chord, 2 of the V chord and finally 2 more of the I chord.
A common variation is to play the IV chord in the second and tenth bars and to play the V chord in the 12th. before starting the progression over. This Progression is sometimes refered to as "quick change."