A mashup (also mashers mesh, mash up, mash-up, blend, bootleg and bastard pop/rock) is a song or composition created by blending two or more pre-recorded songs, usually by overlaying the vocal track of one song seamlessly over the instrumental track of another. To the extent that such works are “transformative” of original content, they may find protection from copyright claims under the “fair use” doctrine of copyright law.
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Mashers History

The practice of assembling new songs from purloined elements of other tracks stretches back to the beginnings of recorded music. If one extends the definition beyond the realm of pop, precursors can be found in musique concrète, as well as the classical practice of (re-)arranging traditional folk material and the jazz tradition of reinterpreting standards. In addition, many elements of mashup culture have antecedents in hip hop and the DIY ethic of punk as well as overlap with the free culture movement.