Blue Book offers a special design of standard format to Enhanced-CD for Multi-media computers according to the features defined in the Red Book and Yellow Book. CD-Extra is an extend disc called Laser Disc, Audio CD+, CD+ or CD Plus. It has two sessions; one session is to store music rails and the other to store data rails. Usually, the CD player will read the first session first, so the music can be played automatically, the computer shall read the data on the second session, and users can also play the music on the first session by software. This technique commonly used in computer game discs, so that stereo music can tie in with the game running smoothly and enjoy the thrill of Computer Entertainment. Nowadays, some discs with music protection, such as CDS disc published by EMI and some Japanese companies that adopt this technology.