These concentric event horizons become degenerate for 2"rQ" = "r"s, which corresponds to an extremal black hole. Black holes with 2"rQ" > "r"s can not exist in nature because if the charge is greater than the mass there can be no physical event horizon (the term under the square root becomes negative). Objects with a charge greater than their mass can exist in nature, but they can not collapse down to a black hole, and if they could, they would display a naked singularity. Theories with supersymmetry usually guarantee that such "superextremal" black holes cannot exist.