The blues is marked by a 12-bar form and relatively specific harmonic content and lyric structure. Within this format, however, blues singers found infinite variety. Rural blues, usually performed by a male singer who accompanied himself on guitar, was exemplified by musicians such as Leadbelly and Robert Johnson. The urban blues, typified by female singers such as Bessie Smith or Mamie Smith who were backed by a pianist of small jazz combo, were recorded and promoted on race records, although white audiences were also interested in the recordings. These two types of blues were dominant, but others were also available to listeners, such as boogie woogie and commercial blues. Blues was one of the earliest genres that became popular on recordings, although print versions of commercial or arranged blues songs also sold well.