"The beginning of a track is signalled when the index marker (index point) is detected.… The marker is automatically recognized by a special sensing device." Following the index marker is the "home address", which indicates the location of this track on the disk, and contains other control information internal to the control unit. A fixed-length "gap" follows the home address. Next, each track contains a "Record 0" (R0), the track descriptor record, which is "designed to enable the entire content of a track to be moved to alternate tracks if a portion of the primary track becomes defective." Following R0 are the data blocks, separated by gaps.