Polychlorinated biphenyls (PCBs) are a family of 209 congeners. Polybrominated diphenyl ethers (PBDEs) are another family of 209 congeners. Any significant quantity of PCBs or PBDEs is by default a blend of multiple molecule types because each molecule forms independently, and chlorine does not strongly select which site(s) it bonds to.
Congeners can refer to similar compounds that substitute other elements with similar valences, yielding molecules having similar structures. Sodium chloride and potassium chloride may be considered congeners. Also potassium chloride and potassium fluoride.