The Blue Book is a compact disc standard developed in 1995 by Philips and Sony. It defines the Enhanced Music CD format (E-CD, also known as CD-Extra, CD-Plus and CD+), which combines audio tracks and data tracks on the same disc. The format was created as a way to solve the problem of mixed mode CDs, which were not properly supported by many CD players. The standard is not freely available and must be licensed from Philips.