Use of copper or copper-clad aluminum (CCA) is more or less universal for speaker wire. Copper has low resistance compared to most other suitable materials. CCA is cheaper and lighter, at the expense of somewhat higher resistance (about the same as copper two AWG numbers up). Copper and aluminum both oxidize, but oxides of copper are conductive, while those of aluminum are insulating. Also offered is Oxygen-free Copper (OFC), sold in several grades. The various grades are marketed as having better conductivity and durability, but they have no significant benefit in audio applications. Commonly available C11000 Electrolytic-Tough-Pitch (ETP) copper wire is identical to higher-cost C10200 Oxygen-Free (OF) copper wire in speaker cable applications. Much more expensive C10100, a highly refined copper with silver impurities removed and oxygen reduced to 0.0005 percent, has only a one percent increase in conductivity rating, insignificant in audio applications.