The blues songs have a musical and lyrical structure. Most musicians associate the musical structure of the blues with a 12-bar repetitive pattern that is played using the I, IV, and V chords. For example, a blues in the key of G uses G (the I chord), C (the IV chord) and D7 (the V chord). The 12-bar structure is G/G/G/G/C/C/G/G/D7/C/G/G. The 12-bar structure is really a more sophisticated version of the blues. Earlier blues forms used only one or two chords. Mississippi Fred McDowell is an example of this style of blues. The lyrical structure is AAB. A vocal phrase is repeated twice and then followed by a conclusion. For example, a verse from Son House's "Death Letter Blues" goes: "I didn't feel so bad until that old evening sun went down," "I didn't feel so bad until that old evening su went down," followed by: "I didn't have no one to wrap my arms around."
The AAB structure reflects the call and response nature of the blues. The call and response characteristic probably has African roots. The singer poses a question in the first part of the lyric, and the concluding part of the lyric offers a response. The interplay between the voice and the musical instruments are also modeled on the call and response model. Blues is a vocal music that relies heavily on improvisation. The guitar player's riffs in between the vocals mimics and responds to the lyrical content. Players such as Blind Lemon Jefferson and Robert Johnson were experts at evoking a mood with their guitars that reflected the lyrical content of the song. The bottleneck style became popular in Delta blues because a bottleneck can easily imitate the nuances of the human voice.
One of the most identifiable characteristics of blues are so-called "blue notes." The blue notes give blues the edge and tension. The easiest way to understand blue notes is to contrast them with the major scale typically used in European music. A C major scale includes the notes C, D, E, F, G, A, B and C. A blue note is made by lowering or raising the pitch of a note a half step. The two blue notes relied on the most in blues is the flatted 3rd and the flatted 7th. In the key of C major, E-flat replaces E and B-flat replaces B. Two other common blue note is the flatted 5th, G-flat in the key of C, and the flatted 9th, D-flat in the key of C. The blue note is the point of departure for jazz musicians. Jazz players alter every note in the scale
On the surface, the lyrical content of blues songs refers to personal relationships and disappointments. On one hand, this is obviously true. However, beneath the surface, blues lyrics reflected the reality of being black in the Jim Crow South. Blues songs can be interpreted as coded critiques of racism, poverty, violence and social oppression.