The sight of a dozen musicians improvising a blues song during an impromptu jam session has wowed many music fans. All of the participants appear to know instinctively when to change chords and how to bring the song to a close without much discussion or practice. In reality, these musicians knew exactly what their ‘improvised’ song would sound like from the start—a it’s standard musical form known as a 12 bar blues progression. The 12 bar blues progression is taught to musicians in the same way that basic time steps are taught to dancers. This enables inexperienced musicians to play along with more experienced musicians and know exactly what chords to play.
Examining the three main elements—twelve bars, blues, and progression—might be helpful in understanding how a standard 12 bar blues progression works.
A measure is a unit of musical time that is divided into twelve bars. Almost all blues music is written in 4/4 time, which means that a quarter note gets a full beat, and a measure is made up of four of these beats. Because measures are denoted by vertical bars in sheet music notation, many musicians refer to them as bars informally. A twelve bar blues progression is twelve measures long in total, but it is usually repeated until the song is finished. A 12 bar blues progression can be modified to only use 8 bars before repeating.
The term “blues” refers to the song’s actual style. Most musicians use a unique rhythm pattern to control their playing in blues music. Despite the fact that blues are written in 4/4 time, the actual rhythm is a syncopated backbeat. Instead of the standard ONE TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO TWO T A 12 bar blues progression’s signature feel and earthy sound comes from this syncopated, driving rhythm.
The chord changes made during a 12 bar blues riff are referred to as progression. In musical terms, the tonic, sub-dominant, and dominant seventh chords are used in most blues songs. A blues song can be in any key, but most musicians agree on E, A, or D as the easiest keys for guitarists to play. When changing chords, musicians follow a standard pattern once the overall key has been determined. The tonic chord of the key is used for the first four measures; for example, if the song is in the key of D, the tonic chord is D major. “Woke up this morning/Wind howling at my door,” the lead singer sings plaintively about his or her life. The band switches to the sub-dominant chord (in this case G major) after the fourth measure, and the singer repeats the line with more urgency: “I said I WOKE UP this MOOORNING/wind HOWLING at my DOOR.” The band then plays two more measures on the original tonic chord (D major).
The final measures are performed and sung in a different way. The singer introduces a new line, “Don’t know if I can make it/Can’t seem to find the floor,” which is a twist on the original: “Don’t know if I can make it/Can’t seem to find the floor.” The band switches to a dominant chord (A major) with an extra note called a seventh added at the same time. The tension created by this added note can only be relieved by returning to the sub-dominant (G major) or tonic (D major). In most 12-bar blues progressions, the band plays one measure of dominant seventh, then one measure of sub-dominant (G major), before returning to the original tonic (D major).
The blues progression is more difficult to explain than to perform. The musicians play the dominant seventh chord for tension, the sub-dominant for some release of that tension, and the tonic to start the progression all over again as the singer begins the new lyric.