Hillbilly music was not widely recorded until the 1920s. Bristol, a city on the Virginia and Tennessee border, was the site of a two-week recording session in 1927 that led to the discovery of the two biggest names in hillbilly music: The Carters and Jimmie Rodgers. The Carters were a trio playing Auto-harp and Guitar, with clear, strong vocals and harmonies, while Rodgers sang a more worldly, blues-influenced music that has been called country blues. Rodgers sold millions of records in the 1930s During this period, hillbilly music became big business, and musicians began endorsing products as well as adding new instruments, like fiddles, banjos, mandolins and Hawaiian steel guitar. Some other important musicians of this era include Gid Tanner & the Skillet Lickers and Charlie Poole & the North Carolina Ramblers.