Many music fans have marveled at the sight of a dozen musicians improvising a blues song during an impromptu jam session. Without much discussion or practice, all the participants seem to know instinctively when to change chords and how to draw the song to a close. In actuality, however, these musicians knew all along what their 'improvised' song would sound like- it's a standard musical form called a 12 bar blues progression. Musicians learn the 12 bar blues progression much like dancers learn basic time steps. This allows amateur musicians to play along with more experienced professionals and know precisely what chords to play.
In order to understand how a standard 12 bar blues progression works, it might be helpful to examine the three main elements- twelve bars, blues, and progression.
Twelve bars refers to a length of musical time called a measure. Almost all blues music is written in 4/4 time, which means a quarter note receives a full beat and four of these beats comprise a measure. In sheet music notation, measures are designated with vertical bars, so many musicians refer to measures informally as bars. The total length of a 12 bar blues progression is twelve measures, although the progression is generally repeated until the song has ended. There are variations of a 12 bar blues progression which only use 8 bars before repeating.
Blues refers to the actual style of the song. Blues music has a unique rhythm pattern which most musicians use to control their playing. Although blues are written in 4/4, the actual rhythm is more of a syncopated backbeat. Instead of the standard ONE two three four/TWO two three four regularity of most 4/4 compositions, blues music uses a driving rhythm: "BOM ba BOM ba BOM ba BOM/BOM ba BOM ba BOM ba BOM". This syncopated, driving rhythm gives a 12 bar blues progression its signature feel and earthy sound.
Progression refers to the chord changes made during a 12 bar blues riff. Most blues songs use three chords- in musical terminology they are the tonic, sub-dominant and dominant seventh. A blues song can be in any key, but generally the musicians agree on keys easiest for guitarists to play, such as E, A, or D. Once the overall key has been determined, musicians follow a standard pattern when changing chords. The first four measures are in the tonic chord of the key- if the song is in the key of D, the tonic chord is D major. The lead singer sings a plaintive lyric about his or her life: "Woke up this morning/Wind howling at my door." After the fourth measure, the band ramps up to the sub-dominant chord (in this case G major) and the singer repeats this line with more urgency: "I said I WOKE UP this MOOORNING/wind HOWLING at my DOOR." The band then returns to the original tonic chord (D major) and plays two more measures.
The last measures are played and sung differently. A new lyric is introduced by the singer which puts a twist on the original line: "Don't know if I can make it/Can't seem to find the floor." At the same time, the band shifts to a dominant chord (A major) with an extra note called a seventh added. This added note creates tension which can only be released by returning to the sub-dominant (G major) or the tonic (D major). In most 12 bar blues progressions, the band plays one measure of the dominant seventh, then steps down to one measure of the sub-dominant (G major) and then finally back to the original tonic (D major).
The blues progression is harder to explain than it is to actually perform. As the singer begins the new lyric, the musicians play the dominant seventh chord for tension, the sub-dominant for some release of that tension and the tonic to start the entire progression over again.