A polychlorinated biphenyl (PCB; CAS number ) is a synthetic organic chemical compound of  chlorine attached to biphenyl, which is a molecule composed of two benzene rings. There are 209 configurations of organochlorides with 1 to 10 chlorine atoms. The chemical formula for a PCB is C₁₂H_10-xCl_x. Of the different PCB arrangements and orientations 130 are used commercially.