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.

Below is an example of a 12-bar blues stanza from "Dust My Broom," as performed by Elmore James, and broken down by bars (measures), beats, chords, and lyrics:



In each 12-bar stanza, the third four-bar segment (in the example above, the 9-12th bars), serves to resolve the previous four-bar segments. The resolution may signal the end of the song or set up another stanza. If the song continues, the transition to the next stanza is known as the turnaround. "Dust My Broom," for example, contains seven 12-bar stanzas, with a turnaround between each.

Not all blues songs follow the 12-bar format, but by understanding this basic musical framework, the listener will gain a deeper understanding and appreciation for all blues music.