Hear the Man in Black and the King share one stage

1. 

   01Gospel

   Hymns at home and in church

   Gospel is Christian music carried by strong lead voices and close harmony, and both men began there. Cash's early memories were dominated by gospel songs and the radio, and decades later he recorded an album of traditional gospel songs. Presley's family attended an Assembly of God church, where he found his first musical inspiration. In the white Southern tradition, gospel was often sung by quartets, a set-up summed up in the phrase four men and a piano.

2. 

   02Country

   The hillbilly music on the radio

   Country music was so often called hillbilly music in the 1940s and 1950s that rockabilly took half its name from the word. Cash, born to poor cotton farmers in Arkansas, sang on a local radio station in high school and went on to make his first name on the country charts. Presley, as a schoolboy in Tupelo, was teased as a kid who played hillbilly music.

3. 

   03Blues

   Call, response and the bent note

   The blues took shape among African Americans in the Deep South around the 1860s. Its building blocks are call and response, the blues scale and set chord patterns, the twelve-bar blues above all, along with the flattened blue notes that give it its ache. The form runs through jazz, rhythm and blues and rock and roll, and Cash's music, his biography notes, embraced it alongside country, folk and gospel.

4. 

   04Rhythm and blues

   The beat Sam Phillips loved

   Rhythm and blues grew up in African American communities in the 1940s, and in its commercial years the usual band was piano, guitars, bass, drums and saxophone. Sun Records' founder, Sam Phillips, loved it. Before Presley, his label had concentrated mainly on African American musicians, because Phillips wanted to bring rhythm and blues to a white audience, and Presley's first records fused it with country.

5. 

   05Rockabilly

   Rock plus hillbilly

   Rockabilly is one of the earliest styles of rock and roll, dating to the early 1950s in the American South. It blends country with rhythm and blues, and strong rhythms, boogie-woogie piano, a twang in the voice and tape echo defined its sound. Presley, playing with the guitarist Scotty Moore and the bassist Bill Black, is called one of its pioneers, and Cash rose to fame in the rockabilly scene in Memphis in the mid-1950s.

6. 

   06Nashville sound

   Strings, choruses and polish

   In the 1950s, producers in Nashville at RCA Victor, Columbia and Decca replaced the fiddles, steel guitar and nasal vocals of honky-tonk with string sections, background voices and pop song structures, partly to win back record buyers lost to rock and roll. The style became known as the Nashville sound. One essayist even dates the start of that era to a Presley record cut in July 1956, by which time RCA Victor held his contract.

Drawn from the English Wikipedia articles on gospel music, Southern gospel, the blues, rhythm and blues, rockabilly, Sun Records and the Nashville sound, and on Johnny Cash and Elvis Presley. Nothing here describes the show's running order.