Spirituals are traditional folk-gospel songs that originated in African-American slave communities during the 19th century. When African slaves were brought to the United States and Christianized, they began to perceive Biblical parallels to their own situation (and were sometimes even encouraged to do so by zealously evangelical whites). Thus, many spirituals address themes of freedom, earthly suffering, and hope for deliverance. They were sung both at worship services and in the fields, and were passed down orally from generation to generation. Given those origins, spirituals are usually performed very simply and written in repetitive, easily remembered structures (for example, many verses in spirituals include a one-line refrain that's sung by the entire group and repeated after each new line of the verse). In that sense, spirituals often bear a resemblance to secular prewar country blues, a music stemming from a similarly rural lineage.