Music is a fluid thing, of course. Genres change and morph over time, especially in the fast-moving world of underground rap. These days, there are any number of countless underground rap subgenres, and it's hard to keep track of them all, even for devoted fans.

With smaller underground rap sub-scenes including everything from "crunkcore" to "backpack rap," it's enough to make an underground rap fan's head spin.

That's especially true since many underground rap subgenres overlap in style and content. For example, a couple of genres who share some, but not all, artists and fans are horrorcore rap and gangster rap.

Both of these underground rap styles favor a hardcore lyrical approach and aggressive beats. But they're not identical. So what is the difference between horrorcore rap and gangster rap? First, a little background on each subgenre individually.

Gangster rap appeared slightly earlier than horrorcore rap, largely in the underground rap scene of the West Coast. Early artists included Ice-T and Schoolly D (who was actually from Philadelphia), and then a little later, N.W.A.

Gangster rap picked up on earlier so-called "hardcore rap," which boasted more confrontational lyrics and aggressive beats than the earliest rap, which was more party-oriented.

Like the name implies, a lot of gangster rap focused on recounting tales of street and gang life, and songs were unabashedly full of violent, but realistic, imagery. Other pioneering gangster rap artists included Too Short and Cypress Hill on the West Coast, and New York's Boogie Down Productions on the East Coast.

Horrorcore rap developed slightly later than gangster rap, although both of the genres' development has overlapped over the years. Horrorcore rap first appeared in the very late '80s and early '90s with groups like the Geto Boys.

Though the Geto Boys were considered at that point to largely be a gangster rap group, they began weaving in songs with horror movie-influenced imagery. Famously they wrote a song called "Chuckie," which, with its lyrics about the infamous murderous doll form the Child's Play movies, was considered one of the first horrorcore rap songs.

Where horrorcore rap distinguished itself was in its over-the-top take on violence. Much of this violence was not particularly realistic and was inspired by the fantasy of movies and television. Soon after the Geto Boys came some of the earliest artists who specializes almost exclusively in horrorcore rap, like Esham in Detroit and Kool Keith and the Flatlinerz in New York.

Just a little later arose one of the most infamous horrorcore rap groups of all time, Detroit's Insane Clown Posse. Taking the persona of murderous clowns, the group was so outrageous that it took horrorcore rap out of underground rap circles and into the mainstream.

While horrorcore rap and gangster rap share certain themes and subjects, they're still not totally the same. Some artists, like DMX and Eminem, have made both gangster rap and horrorcore rap. But the main difference is the degree to which they take their tales. Gangster rap lyrical violence is still rooted somewhat in reality, even if it's composed largely of boasts.

With horrorcore rap, the understanding is that nothing anyone is talking about is meant to be taken literally. This gives horrorcore rap, in a way, many more creative possibilities in underground rap. Because the genre doesn't need to be seen as "real" as gangster rap, horrorcore rap artists can be as disgusting, cartoonish, or ridiculous as they want to be.

They can also draw from a more diverse sonic palette. Whereas gangster rap often requires stripped-down beats, many horrorcore rap artists draw from a wide spectrum of the musical map. Horrorcore artists often incorporate elements of rock, metal, and a number of other styles into their sound.

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These days, gangster rap has started to wane fro the mainstream a little bit, while horrorcore rap continues to gain velocity in the underground rap scene and beyond. One of the main groups helping make this happen is Insane Clown Posse, which has spawned an entire subculture known as juggalos, and which runs its own record label, Psychopathic Records.

To learn more about this seminal horrorcore rap group, visit the official Insane Clown Posse web site at http://www.insaneclownposse.com.