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Physical details of CD
Compact discs are made from a 1.2 mm thick disc of polycarbonate plastic coated with a much thinner layer of Super Purity Aluminium (or rarely, gold, used for its data longevity, such as in some limited-edition audiophile CDs) layer which is protected by a film of lacquer. The lacquer can be printed with a label. Common printing methods for compact discs are silkscreening and offset printing. CDs are available in two sizes. By far the most common is 120 mm in diameter, with a 74-minute audio capacity and a 650 MB data or a 80-minute audio capacity and a 700 MB data (See storage capacity; this form factor has also erroneously been called "CD5" since it is 4 3/4 inches in diameter, about five inches across). Such a standard disc weighs 15 grams. 80 mm discs are also available, a format which is mainly used for audio CD singles in some regions (e.g. Japan), much like the old vinyl single. Each such "miniCD" or "Maxi CD" can hold 21 minutes of music, or 180 MB of data (this form factor has also been called "CD3", since it is about three inches across).
Other unique shapes and smaller form factors have also been sold or given away as promotional items. Examples include Business Card CDs in the shape of a rectangular card and CDs shaped like the map of a country etc, although such discs are not always compatible with all CD players they will work with any machine where the disc is inserted by manually clipping it onto the spindle (the mechanism used in virtually all portable CD players), but may not necessarily be inserted into drives which load the disc from a tray, or pull it into a slot. Irregularly shaped, non rotationally symmetric discs with an offset centre of mass may also cause damaging vibration if played in computer CD drives, which can operate at a much higher rotational velocity than stand-alone audio CD players. Some irregularly shaped discs will work with tray loading CD drives if they include a circular ridge on their underside which centers them on the part of the tray designed to hold 80 mm CDs, assuming the tray has such a feature.
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