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The largest entity on a CD is called a [[Track (optical disc)|track]]. A CD can contain up to 99 tracks (including a data track for [[Mixed Mode CD|mixed mode discs]]). Each track can in turn have up to 100 indexes, though players that still support this feature have become rarer over time. The vast majority of songs are recorded under index 1, with the [[pregap]] being index 0. Sometimes [[hidden track]]s are placed at the end of the last track of the disc, often using index 2 or 3, or using the pregap as index 0 (this latter usage will result in the track playing as the time counter counts down to time 0:00 at the start of the track, index 1.) 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.{{efn|[[Information Society (band)|Information Society]]'s ''[[Hack (album)|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 were carried forward to the DVD format as title and chapter, respectively.
Tracks, in turn, are divided into timecode frames (or sectors), which are further subdivided into channel-data frames.
==== Frames and timecode frames ====
{{Main|Track (optical disc)#Sector structure}}
The smallest entity in a CD is a channel-data ''frame'', which consists of 33 bytes and contains six complete 16-bit stereo samples: 24 bytes for the audio (two bytes
On a ''Red Book'' audio CD, data is addressed using the ''MSF scheme'', with [[timecode]]s expressed in minutes, seconds and another type of ''frames'' (mm:ss:ff), where one frame corresponds to 1/75 of a second of audio: 588 pairs of left and right samples. This timecode frame is distinct from the 33-byte channel-data frame described above, and is used for time display and positioning the reading laser. When editing and extracting CD audio, this timecode frame is the smallest addressable time interval for an audio CD; thus, track boundaries only occur on these frame boundaries. Each of these structures contains 98 channel-data frames, totaling 98 × 24 = 2,352 bytes of music. The CD is played at a speed of 75 frames (or sectors) per second, thus 44,100 samples or 176,400 bytes per second.
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