At its outset, alternative metal was a style united by its nonconformist sensibility rather than any immediately classifiable sound. Heavy metal was at the core of the music, but the bands were too offbeat and their influences too eclectic to fit into the thrash underground, so their main audiences were mostly alternative fans who liked heavy guitar rock. However, after grunge helped make alternative metal more palatable to mass audiences, it became the most popular style of heavy metal in the '90s, particularly when more aggressive bands began standardizing its sound. That approach was a far cry from alternative metal's early days in the late '80s, when it represented the least categorizable heavy music around.