The most common musical form of blues is the 12-bar blues. The term "12-bar" refers to the number of measures, or musical bars, used to express the theme of a typical blues song. Nearly all blues music is played to a 4/4 time signature, which means that there are four beats in every measure or bar and each quarter note is equal to one beat.
A 12-bar blues is divided into three four-bar segments. A standard blues progression, or sequence of notes, typically features three chords based on the first (written as I), fourth (IV), and fifth (V) notes of an eight-note scale. The I chord dominates the first four bars; the IV chord typically appears in the second four bars (although in the example below, Elmore James introduces it in the first four bars); and the V chord is played in the third four bars.
The lyrics of a 12-bar blues song often follow what's known as an AAB pattern. "A" refers to the first and second four-bar verse, and "B" is the third four-bar verse. In a 12-bar blues, the first and second lines are repeated, and the third line is a response to them—often with a twist.