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The Blue Book is a standard for audio CDs developed by Sony and Philips that allows for extra content on a disc. This multimedia content can be viewed on a personal computer with an optical drive. These discs are known as "enhanced CDs" because they combine audio and data content on the same disc.
The Blue Book standard is known by many other terms, including CD-Extra, CD-Plus, CD+, Enhanced Music CD and E-CD.
The Blue Book was a follow-on to the existing CD audio standard, known as the Red Book, developed by Sony and Philips in 1995. They were intended to provide both CD audio and extra multimedia content on the same disc.
They were most widely issued in the mid-late 1990s, marketed as "enhanced CDs." For example, an enhanced music CD might include music videos, lyrics or links to an artist's website when the disc is inserted into a computer. Much of this content was built with HyperCard. They are much less common nowadays, with record labels opting to put extra content on DVDs instead.