Blues is the music that launched a thousand garage bands. Providing musicians with a shorthand vocabulary, the form takes a standard collection of instruments and gives accessible form to both music and lyrics. Its simple and easily learned elements are a platform for collaboration and improvisation while serving as a gateway to rock and jazz music, connecting these to the field hollers from the cotton fields of the Deep South.
As a musical style steeped in folk tradition, precise origins of the blues are difficult to define. Musicologists generally attribute a relationship to the music of the central Sudanic region of Africa, which crossed the Atlantic with the slave trade. As portable recording technology emerged in the early 20th century, archivists began to capture blues players' music. Commercially released race records fixed and simplified the forms and conventions and allowed the spread of blues north, beyond its roots in the Southern states.
Harmonically, blues songs use the I, IV, V chords in a given scale. For example, a song in the key of A would use A, D and E chords -- the first, fourth and fifth notes of the A scale. The rhythm is structured into a 12-bar, 48-beat pattern that often repeats for the duration of the song. Melodically, flattened notes blur the feeling of major and minor scales, usually the third and fifth notes of the scale. These blue notes are common with bending and sliding on guitars during solos.
While blues rose from strictly vocal music in the cotton fields, acoustic guitars and banjos were the earliest instruments to accompany solo blues singers. As musicians moved north to follow factory jobs in urban centers, electric guitars came to the fore, defining both the Chicago and, later, British schools of blues. The harmonica, or blues harp, links back to the earliest days of the genre as a portable and easy-to-play instrument. As blues bands developed, standard rhythm instruments included a drum kit and bass, with electric bass emerging in the 1950s. Pianos and organs became more prominent in this period as well. Another common instrument for blues soloing is the saxophone.
Blues has always wandered from the I-IV-V, 12-bar structure, and many songs with "blues" in the title incorporate popular song forms rather than strict blues construction. The "feeling blue" idea of lyrical themes is not universal as some blues songs have up-tempo, dance elements.