<P> The writer Ed Morales claimed that Yoruba mythology played a part in early blues, citing Robert Johnson's "Cross Road Blues" as a "thinly veiled reference to Eleggua, the orisha in charge of the crossroads". However, the Christian influence was far more obvious . The repertoires of many seminal blues artists, such as Charley Patton and Skip James, included religious songs or spirituals . Reverend Gary Davis and Blind Willie Johnson are examples of artists often categorized as blues musicians for their music, although their lyrics clearly belong to spirituals . </P> <P> The blues form is a cyclic musical form in which a repeating progression of chords mirrors the call and response scheme commonly found in African and African - American music . During the first decades of the 20th century blues music was not clearly defined in terms of a particular chord progression . With the popularity of early performers, such as Bessie Smith, use of the twelve - bar blues spread across the music industry during the 1920s and 30s . Other chord progressions, such as 8 - bar forms, are still considered blues; examples include "How Long Blues", "Trouble in Mind", and Big Bill Broonzy's "Key to the Highway". There are also 16 - bar blues, such as Ray Charles's instrumental "Sweet 16 Bars" and Herbie Hancock's "Watermelon Man". Idiosyncratic numbers of bars are occasionally used, such as the 9 - bar progression in "Sitting on Top of the World", by Walter Vinson . </P> <Table> <Tr> <Td> Chords played over a 12 - bar scheme: </Td> <Td> Chords for a blues in C: </Td> </Tr> <Tr> <Td> <Table> <Tr> <Td> I </Td> <Td> I or IV </Td> <Td> </Td> <Td> I7 </Td> </Tr> <Tr> <Td> IV </Td> <Td> IV </Td> <Td> </Td> <Td> I7 </Td> </Tr> <Tr> <Td> V </Td> <Td> V or IV </Td> <Td> </Td> <Td> I or V </Td> </Tr> </Table> </Td> <Td> <Table> <Tr> <Td> </Td> <Td> C or F </Td> <Td> </Td> <Td> C7 </Td> </Tr> <Tr> <Td> </Td> <Td> </Td> <Td> </Td> <Td> C7 </Td> </Tr> <Tr> <Td> </Td> <Td> G or F </Td> <Td> </Td> <Td> C or G </Td> </Tr> </Table> </Td> </Tr> </Table> <Tr> <Td> Chords played over a 12 - bar scheme: </Td> <Td> Chords for a blues in C: </Td> </Tr>

Where were the first commercial blues recordings made