A contrafact is a new musical composition built out of an alreay existing one, most often a new melody overlaid on a familiar harmonic structure. As a compositional device, it was of particular importance in the 1930s/1940s development of bop, since it allowed jazz musicians to create new pieces for performance and recording on which they could immediately improvise, without having to seek permission or pay publisher fees for copyrighted materials (while melodies can be copyrighted, the underlying harmonic structure cannot be).