Twelve-bar blues is the most popular, and recognizable, example of a blues chord structure. This structure relies on a chord progression of I, IV, V. This means that regardless of the key the music is in, the tonic of each of the three chords will be the first, fourth, and fifth note of the scale. The most typical example of a 12-bar blues progression is 16 beats of the I chord, eight beats of the IV chord, eight more beats of the I chord, four beats of the V chord, four more beats of the IV chord and finally eight more beats of the I chord. If each beat is a quarter note, this totals 48 beats, or 12 measures.
The melody of most blues tunes features a flattened third, fifth and seventh within a major scale. These notes are commonly referred to as "blue notes." The famous jazz record label Blue Note Records takes its name from this term. Blue notes are a major part of what gives blues its distinguishable sound.
The lyrics of a typical blues song consist of three lines. The first two lines will generally be very similar and occupy the first eight bars of the chord progression. The final four bars of the progression are devoted to the third line, which is often a response to the reason for the first two lines.
Blues instrumentalists typically favor simplistic guitar, bass and drum playing, although it's important to realize that there is plenty of room in blues music for virtuosic play. This is especially true of the solos that will typically follow one or two iterations of the 12-bar chord progression. During verses, players of stringed instruments will typically use plenty of open chords, including strings that are not fretted.
The structure of blues music has influenced many other genres of music, such as folk, rock and jazz. Jazz musicians such as Louis Armstrong and Miles Davis heavily relied on blues chord progressions to form the basic structure of their writing. Rock bands old and new, such as Led Zeppelin or the White Stripes, display clear blues influences in their music, and have even been known to include covers of traditional blues songs in their sets.