text: Audio compact cassettes use magnetic tape of three major types which differ in fundamental magnetic properties, the level of bias applied during recording, and the optimal time constant of replay equalization. Specifications of each type were set in 1979 by the International Electrotechnical Commission (IEC): Type I (IEC I, ferric or normal tapes), Type II (IEC II, or chrome tapes), Type III (IEC III, ferrichrome or ferrochrome), and Type IV (IEC IV, or metal tapes). Type 0 was a non-standard designation for early compact cassettes that did not conform to IEC specification.
  By the time the specifications were introduced, Type I included pure gamma ferric oxide formulations, Type II included ferricobalt and chromium(IV) oxide formulations, and Type IV included metal particle tapes—the best-performing, but also the most expensive. Double-layer Type III tape formulations, advanced by Sony and BASF in the 1970s, never gained substantial market presence. 
  In the 1980s the lines between three types blurred. Panasonic developed evaporated metal tapes that could be made to match any of the three IEC types. Metal particle tapes migrated to Type II and Type I, ferricobalt formulations migrated to Type I. By the end of the decade performance of the best Type I ferricobalt tapes (superferrics) approached that of Type IV tapes; performance of entry-level Type I tapes gradually improved until the very end of compact cassette production.