While it's common knowledge that blues were born in the South, exactly how and when it took on an organized song form is not. Not much was known about the blues at all prior to the 1930s. That's when the father and son team of John and Alan Lomax began recording blues artists for the Library of Congress folk music archives. Some of the recordings are of artists who gave their interpretations of the proto-blues of the late 1800s.
Modern blues have definitive musical elements. Songs are built around 12-bar sections, though 8 and 16-bar variations are common. Early vocal blues had far less form and was often performed without any bar counts at all. Flattened thirds, fifths and sevenths of musical scales are used extensively, and they provide a dissonant or "worried" feeling. It is usually embodied in the "crying" sound of blues harmonica or bent guitar notes.
The content of blues lyrics is organized loosely into three-line verses. The first two are the same, or variations of the same line, and the third line resolves or addresses the problem raised in the first two. This may spring from the call-and-response form, which is another thread shared with African music. Many blues themes, such as loss, hardship, loneliness and heartbreak, recur in standard phrases that appear often in different songs.
Certain instruments lend themselves naturally to the blues style. Guitar and harmonica in particular were portable and allowed pitch bending, making it easy to create blue notes. As blues became urbanized, the saxophone crossed over from jazz and became a featured instrument. The piano took a high profile in stride, barrelhouse and New Orleans blues.
Blues music took on electric instruments as the black population moved north to follow employment. Chicago blues became the most prominent. New Orleans and Memphis embellished blues forms in jazz and Dixieland. The British Invasion of the mid 1960s introduced an English rock take on the blues, which in turn exposed it to the white population of America. In the later '60s, blues-rock, which stemmed from Chicago and Detroit blues influences, became its own genre.