A mass storage medium for analog and digital data. Its most common configuration is its use in audio cassettes, used for distributing music, speech, or for other uses, such as in answering machines. Essentially, magnetic tape is a metallic or plastic strip coated with a magnetic substance such as an iron oxide (usually ferric oxide—Fe2O3—obtained from hematite or magnetite). The electromagnetic record heads alter the magnetic polarity of different portions of the tape, and the play head converts the pattern of magnetized spots to an electronic signal which can be converted to sound waves or computer data. Magnetic tape is used as a storage medium for computer information, but the ability to only access the tape serially (i.e., from beginning to end, not at random, as with other magnetic media) makes it impractical for general use. Magnetic tape is also used for the strips on magnetic cards such as credit cards. Magnetic tape stored in a hard plastic cartridge is known as tape cartridge.