Underground hip hop is an umbrella term for hip hop music outside the general commercial canon. It is typically associated with independent artists, signed to independent labels or no label at all. Underground hip hop is often characterized by socially conscious, positive, or anti-commercial lyrics. However, there is no unifying or universal theme – Allmusic suggests that it "has no sonic signifiers". "The Underground" also refers to the community of musicians, fans and others that support non-commercial, or independent music. Music scenes with strong ties to underground hip hop include alternative hip hop and horrorcore. Many artists who are considered "underground" today were not always so, and may have previously broken the Billboard charts.