The "Green Book", formally known as the "CD-i Full Functional Specification", is a CD standard announced in 1986 by Philips and Sony that defines the format for interactive, multimedia compact discs designed for CD-i players. The standard was originally not freely available and had to be licensed from Philips. However, the 1994 version of the standard was eventually made available free by Philips.
CD-i discs conform to the Red Book specification of audio CDs (CD-DA). Tracks on a CD-i's program area can be CD-DA tracks or CD-i tracks, but the first track must always be a CD-i track, and all CD-i tracks must be grouped together at the beginning of the area. CD-i tracks are structured according to the CD-ROM XA specification (using either Mode 2 Form 1 or Mode 2 Form 2 modes), and have different classes depending on their contents ("data", "video", "audio", "empty" and "message"). "Message" sectors contain audio data to warn users of CD players that the track they are trying to listen to is a CD-i track and not a CD-DA track. The CD-i specification also specifies a file system similar to (but not compatible with) ISO 9660 to be used on CD-i tracks, as well as certain specific files that are required to be present in a CD-i compatible disc.
The CD-i Ready format is a type of bridge format, also designed by Philips, that defines discs compatible with CD Digital audio players and CD-i players. This format puts CD-i software and data into the pregap of Track 1.
The CD-i Bridge format, defined in Philips' White Book, is a transitional format allowing bridge discs to be played both on CD-ROM drives and on CD-i players.