Early mash ups simply overlaid the vocals from one song with the backing tracks from another. The Freelance Hellraiser played the vocal from Christina Aguilera's "Genie in a Bottle" over the guitar from The Strokes' "Hard to Explain" to create "A Stroke of Genius." Richard X mixed Whitney Houston's "I Wanna Dance with Somebody" with Kraftwerk's "Numbers"; his later mash up of Adina Howard's "Freak Like Me" and Gary Numan's "Are Friends Electric?" was so popular that the U.K. pop band The Sugababes recorded a version of it.
Mash up artists later moved on to producing full albums of mash ups. Danger Mouse remixed The Beatles' "White Album" into a set of hip-hop instrumentals, then overlaid the acapellas from Jay-Z's "Black Album" to create "The Grey Album." Because Danger Mouse owned none of the rights to this music, the album was never officially released, but was wildly popular on Internet file-sharing sites. The artist Girl Talk has released several full albums of mash ups. His mash ups usually combine five or six different songs into one: a typical Girl Talk production might include samples from Eminem, N.W.A., Jane's Addiction and David Banner, all played at the same time.
The legality of making mash ups is murky. While record companies claim that sampling the music they own is a violation of copyright, the artists claim that mash ups fall under the definition of fair use. Copyright law as it related to mash ups has been essentially determined on a case-by-case basis. EMI blocked Danger Mouse from releasing The Grey Album; however, as of March, 2011, Girl Talk has not been sued for using unauthorized samples.
The easiest way to make your own mash up is to play two songs in two instances of a media player, then record the results. You can improve on this relatively unsophisticated approach by using a virtual DJ mixer on your computer to loop the parts of the songs you want to use. For the greatest control over the final mash up, use an audio editor or digital audio workstation to cut up the songs and overlay the different parts. Always respect copyright laws when making your own mash ups.