Blues is a music genre based on the use of the blues chord progressions and the blue notes. Though several blues forms exist, the twelve-bar blues chord progressions are the most frequently encountered. Blue notes are sung or played at a slightly lower pitch than that of the major scale for expressive purposes.

Blues emerged at the end of the 19th century as an accessible form of self-expression in African-American communities of the United States from spirituals, work songs, field hollers, shouts and chants, and rhymed simple narrative ballads. The use of blue notes and the prominence of call-and-response patterns in the music and lyrics are indicative of African influences. The blues influenced later American and Western popular music, as the blues form became a basic pattern of jazz, rhythm and blues, bluegrass and rock and roll.

In the 1960s and 1970s, blues evolved into a hybrid form called blues rock. In the 1990s, punk blues appeared as an outgrowth of the blues rock and punk movements. The term "the blues" refers to the "the blue devils", meaning melancholy and sadness; an early use of the term in this sense is found in George Colman's one-act farce Blue Devils (1798).

Though the use of the phrase in African American music may be older, it has been attested to since 1912, when Hart Wand's "Dallas Blues" became the first copyrighted blues composition. In lyrics the phrase is often used to describe a depressed mood. During the first decades of the twentieth century, blues music was not clearly defined in terms of a chord progression.

There were many blues in 8-bar form, such as "How Long Blues", "Trouble in Mind", and Big Bill Broonzy's "Key to the Highway". Idiosyncratic numbers of bars are also encountered occasionally, as with the 9-bar progression in "Sitting on Top of the World". The basic twelve-bar lyric framework of a blues composition is reflected by a standard harmonic progression of twelve bars in 4/4 or (rarely) 2/4 time.

Slow blues are often played in 12/8 (4 beats per measure with 3 subdivisions per beat).[citation needed]

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