CD Logical structure

The largest entity on a CD is called a track. A CD can contain up to 99 tracks (including a data track for mixed mode discs). Each track can in turn have up to 100 indexes, though players which handle this feature are rarely found outside of pro audio, particularly radio broadcasting. The vast majority of songs are recorded under index 1, with the pre-gap being index 0. Sometimes hidden tracks are placed at the end of the last track of the disc, often using index 2 or 3. This is also the case with some discs offering "101 sound effects", with 100 and 101 being indexed as two and three on track 99. The index, if used, is occasionally put on the track listing as a decimal part of the track number, such as 99.2 or 99.3. (Information Society's Hack was one of very few CD releases to do this, following a release with an equally-obscure CD+G feature.) The track and index structure of the CD carried forward to the DVD as title and chapter, respectively.