A vocal and instrumental form of music based on the use of the blue notes and a repetitive pattern that most often follows a 12-bar structure. It evolved in the United States in the communities of former African slaves, from spirituals, praise songs, field hollers, shouts and chants. The use of blue notes and the prominence of call-and-response patterns in the music and lyrics are indicative of the blues' West African pedigree. The blues influenced later American and Western popular music, as it became part of the genres of ragtime, jazz, bluegrass, rhythm and blues, rock and roll, hip-hop, country music and pop songs.

Blues has evolved from an unaccompanied vocal music of poor black labourers into a wide variety of styles and subgenres, with regional variations across the United States and, later, Europe and Africa. The musical forms and styles that are now considered the blues as well as modern country music arose in the same regions during the 19th century in the southern United States.

Recorded blues and country can be found from as far back as the 1920s, when the popular record industry developed.At the time, there was no clear musical division between blues and country, except for the race of the performer, and even that sometimes was documented incorrectly by record companies. In the 1920s, the blues became a major element of African American and American popular music and evolved from informal performances in bars to entertainment in theatres, reaching white audiences via W.C. Handy's arrangements and the classic female blues performers.

The first blues recordings from the 1920s were in two categories: a traditional, rural country blues and more polished 'city' or urban blues.

There were many regional styles of country blues in the early 20th century. The (Mississippi) Delta blues was a rootsy sparse style with passionate vocals accompanied by slide guitar. Robert Johnson, who was little-recorded, combined elements of both urban and rural blues. Along with Johnson, influential performers of this style were his predecessors Charley Patton and Son House.

As the recording industry grew, country blues performers like Patton, Blind Lemon Jefferson, Son House and Blind Blake became popular in the African American community. Jefferson was one of the few country blues performers to record widely and may have been the first to record the slide guitar style, which became an important part of the Delta blues.

Many blues musicians based in the South moved to Chicago in the late 1930s or early 1940s and became part of the city or urban blues movement which blended country music and electric blues and ws generally more elaborate. Boogie-woogie was another important style of 1930s and early 1940s urban blues. While the style is often associated with solo piano, boogie-woogie was also used to accompany singers and, as a solo part, in bands and small combos.

In the 1940s, the jump blues style developed, influenced by big band music and using saxophone or other brass instruments and the guitar in the rhythm section to create a jazzy, up-tempo sound with declamatory vocals. Jump blues tunes by Louis Jordan and Big Joe Turner, based in Kansas City, influenced the development of later styles such as rock and roll and rhythm and blues.

The smooth Louisiana style of Professor Longhair and, more recently, Dr. John blends classic rhythm and blues with blues styles.

After World War II and in the 1950s, as African Americans moved to the Northern cities, new styles of electric blues music became popular in cities such as Chicago, Detroit and Kansas City. Electric blues used amplified electric guitars, electric bass, drums and harmonica. Chicago in particular became a center for electric blues in the early 1950s. Chicago blues is influenced to a large extent by the Mississippi blues style, because many performers had migrated from the Mississippi region.

In the 1950s, blues had a huge influence on mainstream American popular music and in particular on the development of rockabilly with artists such as Bo Diddley and Chuck Berry.

Chicago blues also influenced Louisiana's zydeco music, with artists such as Clifton Chenier. By the late 1950s, the swamp blues genre developed near Baton Rouge, with performers such as Slim Harpo. Swamp blues has a slower pace and a simpler use of the harmonica than the Chicago blues style.

By the beginning of the 1960s, genres influenced by African-American music such as rock and roll and soul were part of mainstream popular music. Caucasian performers had brought African-American music to new audiences, both within the US and abroad.

In the UK, bands emulated US blues legends and UK blues-rock-based bands such as Fleetwood Mac, John Mayall & the Bluesbreakers, the Rolling Stones, the Yardbirds, Cream and Led Zeppelin had an influential role throughout the 1960s performing material based on the Delta or Chicago blues traditions.

Since the 1980s, blues has continued in both traditional and new forms. In particular, there has been a resurgence of interest in the blues among a certain part of the African-American population, particularly around Mississippi and other deep South regions - often referred to as soul blues. The Texas blues-rock style emerged using guitars in both solo and rhythm roles with major artists such as Stevie Ray Vaughan, the Fabulous Thunderbirds and ZZ Top.