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A Compact Cassette, more commonly known as Audio Tape and Audio Cassette, and known by some European Countries as Musicassette (MC) was a previous standard for recording audio as well as data. There were many forms of tape-storage for data and music over several decades, but a Compact Cassette is what is commonly refered to as Tape. Compact Casettes were most prevelant in the consumer market between 1978 and 1998. Tape, much like Vinyl Records and VHS is alleged to be (as of 2016) making a comeback in the hobbyist market, especially for Vaporwave Musicians.
What most plebs don't know is that Audio Cassette had many different formats, just like VHS. These were four primary different types, based on the material used. These were (in order of quality) Ferro, Chrome, Ferro-Chrome, and Metal, otherwise known as Types I-IV. Type III was short-lived. People recall music on Tape as being poor in quality due to the music industry releasing audio on type I and II only due to affordability.
Tapes for data were known professionally as SAM, or Sequential Access Memory (compare to RAM: Random Access Memory).
Early versions of Dolby were on Cassettes, but require a Dolby-compatible machine. The last versions of Dolby that were supported on cassette were rarer due to CDs gaining in popularity, so you may find only older systems at thrift stores supporting older systems of Dolby.
The Dolby systems avalible for tape were Dolby B, C, S, and HX-Pro. Dolby S was a rarity in the American Market.