Blues music

Blues is the name given to both a musical forms and music genre that originated in African-American communities of primarily the "Deep South" of the United States at the end of the 19th century from spiritual, work songs,field holder , shouts, and rhymed simple narrative ballads

in blues form, ubiquitous in jazz, rhythm and blues, and rock and roll is characterized by specific chord progressions, of which the 12 bar blues chord progression is the most common. 

The term " the blues" refers to the "blues devils" which mean melancholy and sadness  an early use of the term in this sense is found in George Colman's one-act farce Blue Devils (1798).

The basic 12-bar lyric framework of a blues composition is reflected by a standard harmonic progression of 12 bars in a 4/4 time signature. The blues chords associated to a 12 bar blues are typically a set of three different chords played over a 12-bar scheme.

The lyrics of early traditional blues verses probably often consisted of a single line repeated four times; 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.

Blues musical styles, forms (12-bar blues), melodies, and the blues scale have influenced many other genres of music, such as rock and roll, jazz, and popular music.