The origins of the blues are also closely related to the religious music of the Afro-American community, the spirituals.
The first appearance of the blues is often dated to after the ending of slavery and, later, the development of juke joints.
Blues shuffles or walking bass reinforce the trance-like rhythm and form a repetitive effect known as the groove.
Blues as a genre is also characterized by its lyrics, bass lines, and instrumentation.
Other chord progressions, such as 8-bar forms, are still considered blues; examples include "How Long Blues", "Trouble in Mind", and Big Bill Broonzy's "Key to the Highway".
There are also 16-bar blues, such as Ray Charles's instrumental "Sweet 16 Bars" and Herbie Hancock's "Watermelon Man".
Early blues frequently took the form of a loose narrative, often relating the racial discrimination and other challenges experienced by African-Americans.
Many elements, such as the call-and-response format and the use of blue notes, can be traced back to the music of Africa.
It was only in the first decades of the 20th century that the most common current structure became standard: the so-called AAB pattern, consisting of a line sung over the four first bars, its repetition over the next four, and then a longer concluding line over the last bars. Handy wrote that he adopted this convention to avoid the monotony of lines repeated three times.
The blues form, ubiquitous in jazz, rhythm and blues and rock and roll, is characterized by the call-and-response pattern, the blues scale and specific chord progressions, of which the twelve-bar blues is the most common.
Blue notes (or "worried notes"), usually thirds or fifths flattened in pitch, are also an essential part of the sound.
Blues subgenres include country blues, such as Delta blues and Piedmont blues, as well as urban blues styles such as Chicago blues and West Coast blues.
World War II marked the transition from acoustic to electric blues and the progressive opening of blues music to a wider audience, especially white listeners.