A X is a song or composition created from the combination of the music from one song with the a cappella (a capella = vocals) from another.
In full swing at the end of the 20th century, Xs have been described positively as "ultimate post-modern pop song[s]" or "'culture jamming in its purest form'" and "display an analytical impulse...we actually hear how these songs resonate with one another, comment upon and affect one another...in a way, the music speaks for itself". They have also been described negatively as "the logical extension of the sampling fever of the '80s taken to its dumbest extreme". Due to the questionable legal status of Xs and little chance of profits "there's a real punk rock attitude attached to the movement".
Also, "The most interesting and entertaining phenomenon of the MP3 libraries on peer-to-peer systems is the availability of Xs - new compositions created by combining the rhythm tracks of one song and the vocal tracks of another. It is merely the latest incarnation of a widely shared, deeply embedded cultural habit of cultural recombination across time and space."

X is also known by the following names
* Bootlegs (mostly in Europe)
* Boots (but not Booty which is a branch of Electro)
* Y (as in the combined songs are unofficial)
* Blends
* Cutups (or cut ups, a term originally coined by William S. Burroughs to describe some of his literary experiments that involved literally "cutting up" different texts and rearranging the pieces to create a new piece.)
* Powermixing (Usually the pace has to be sped up to allow for more song to be played and thus cannot play any single blend for the full length of the song)

Give me X and Y