"Rez" is a non-album track by Underworld, originally released in 1993 in the UK. It became one of their biggest club/live anthems despite the fact it could not be found on an album. However, it did appear on many compilations, b-sides, and even appeared on a bonus disc for Second Toughest in the Infants (1996). The first promo release of the single was pressed onto pink vinyl, and is highly collectible as it has another non-album track, Why, Why, Why. Due to this track's obscurity, there are, incidentally, many bootlegs of it. The more common release has "Cowgirl" on it. There is also a limited white vinyl pressing available in the US.

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